Sting Morphology and Frequency of Sting Autotomy Among Medically Important Vespids (Hymenoptera: Vespidae) and the Honey Bee (Hymenoptera: Apidae) LORRAINE MULFINGER, JOHN YUNGINGER,1 WILLIAM STYER,2 MILES GURALNICK, AND THOMAS LINTNER Vespa Laboratories, Inc., Spring Mills, Pennsylvania 16875
J. Med. Entomol 29(2): 325-328 (1992)
ABSTRACT Data from clinical sting challenge investigations indicate that certain species of yellowjackets experience sting apparatus autotomy with surprisingly high frequency. However, a retrospective survey of vespid collectors strongly supports the supposition that the frequency of yellowjacket sting autotomy seen in clinical situations is not representative of the frequency experienced under field conditions. Examination by electron microscopy of the sting apparatuses of several vespid species and that of Apis mellifera L., the honey bee, revealed previously unreported structural variations between apid and vespid aculei which likely contribute to differences in sting autotomy rates observed between the honey bee and the social wasps. Specifically, when the lancets of a vespid aculeus are in a retracted position, the width of the smooth-edged stylet extends beyond the barbed edges of the lancets, forming a protective sheath. By contrast, all honey bee aculei possess stylets of insufficient width to shroud the barbs of retracted lancets, thus allowing the barbs to be completely exposed. Additionally, the dorsal surface of all vespid stylets are smooth in contrast to the dorsal surface of honey bee stylets, which support from one to three rows of paired barbs. The exposure of barbs on retracted honey bee lancets and the presence of additional barbs on the dorsal tip of the stylet would make withdrawal of a honey bee aculeus from a victim's flesh more difficult than withdrawal of a vespid aculeus, in which the barbs of retracted lancets are shielded and no dorsal barbs are present. KEY WORDS Insecta, Hymenoptera, sting autotomy, sting morphology
I T HAS LONG BEEN ACCEPTED among aculeate hymenopterists, as well as allergists trained in insect sting hypersensitivity, that the sting autotomy (self-amputation of sting apparatus) rate of the honey bee, Apis mellifera L., approaches 100%, whereas the sting autotomy rate of other medically important hymenopterans, principally of the family Vespidae, is very low. However, a recent report cites an unusually high frequency of sting autotomy with the eastern yellowjacket, Vespula maculifrons (Buysson). During clinical sting challenges performed to evaluate the efficacy of hymenopteran venom immunotherapy, 51 of 79 V. maculifrons workers reportedly experienced sting autotomy (Greene et al. 1989). A number of field-experienced Hymenoptera r i.•ii.r J i.u u • u r specialists round the high frequency of sting autotomy noted by Greene etal. (1989) to be inconsistent with personal experiences. Few had ever experienced stings by yellowjackets that re-
suited in autotomy of the sting apparatus. In an effort to clarify the apparent discrepancy between the frequencies of sting autotomy observed during clinical sting challenges and field sting incidents, the frequencies of sting autotomy for honey bees; yellowjackets, Vespula spp.; aerial yellowjackets, Dolichovespula arenaria (F.); baldfaced hornets, Dolichovespula maculata (L.); and paper wasps, Polistes spp. were evaluated in both field and clinical situations. In addition, aculei from A. mellifera and five different vespid species were examined by scanning electron microscopy (SEM) to determine what external morphological differences exist, A»
. . i j X/f . u i Materials and Methods
The frequencies of sting autotomy among honey bees, yellowjackets, aerial yellowjackets, baldfaced hornets, and paper wasps under "field" conditions were evaluated by means of a questionnaire addressed to 50 experienced 1 Research Laboratory for Allergic Diseases, The Mayo j questionnaire was a retroc i( c o l l e c t o r s > T h e
Clinic, Rochester, Minn. 55905.
.
2 Ohio Agricultural Research and Development Center, Ohio State University, Wooster, Ohio 44691.
i • i
i
j j . -i
r J_I
spective Survey which asked details ot the respondent's last five sting encounters with the
0022-2585/92/0325-0328$02.00/0 © 1992 Entomological Society of America
326
Vol. 29, no. 2
JOURNAL OF MEDICAL ENTOMOLOGY
aforementioned species. The details included the anatomical site of each sting, whether the sting was inflicted on bare skin versus through clothing or protective netting, the identity of the hymenopteran (honey bee, yellowjacket, aerial yellowjacket, bald-faced hornet, or paper wasp), whether the sting was interrupted by swatting or brushing away the insect, and whether the insect experienced sting autotomy at the site. The collectors were also asked to identify, if possible, the species of Vespula when the insect belonged to that genus. The frequencies of sting autotomy during clinical hymenopteran sting challenges were assessed by the Research Laboratory for Allergic Diseases of the Mayo Clinic, Rochester, Minn. Records were kept on the frequencies of autotomy among various hymenopteran species during sting challenges performed between 1980 and 1989. V. maculifrons was the only yellowjacket species tested in clinical sting challenges except in 1983, when several challenges were performed with V. flavopilosa Jacobson. Challenges were executed using a pair of forceps to hold an insect in contact with the upper arm of a human subject until a sting was inflicted. The insect was removed as soon as the subject felt a sting. Insects prepared for SEM were supplied and identified by Vespa Laboratories, Inc. (Spring Mills, Pa.). The insects were quick-frozen upon collection and stored at — 20°C to avoid deterioration. The sting apparatus from thawed specimens was carefully dissected, placed in 70% ethanol, then dehydrated using an ethanol-freon TF dehydration procedure (Liepens & DeHaven 1978). Once dried, the aculei were attached to specimen stubs using low-resistance silver paint (E. Fullam, Latham, N.Y.) and then sputtercoated with approximately 15 nm of platinum using an E5100 series Polaron coater. Specimens were viewed and photographed with an ISI40 scanning electron microscope at magnifications ranging from 20 to 5,000 x. A minimum of five insects of each species was examined. Results The frequency of sting autotomy under field sting conditions for each group of the medically important Hymenoptera was established from the survey of collectors (Table 1). The tabulation included only those stings delivered directly into unclothed skin and without interruption by swatting or brushing away the insect. The sting autotomy rates noted for yellowjackets, aerial yellowjackets, baldfaced hornets, and paper wasps were all low, and there was no suggestion that the rate for any one of these groups was significantly higher than the others. However, as expected, the autotomy rate for honey bees was
Table 1. Sting autotomy rates during uninterrupted field stings Insect
No. stings
No. autotomies
% autotomy
Vespula spp. (incl. V. maculifrons) V. maculifrons only D. arenaria D. maculata Polistes spp. A. mellifera
111 14 12 20 33 89
5 1 1 1 2 71
4.5 7.1 8.3 5.0 6.1 79.8"
"X1 with Yates' correction = 149.4; P < 0.001.
significantly higher than the rates noted for the social wasps (P < 0.001). The frequency of yellowjacket sting autotomy during the past 9 yr of clinical sting challenges performed at the Mayo Clinic was tabulated from clinic records on a yearly basis (Table 2). Yellowjackets used in these sting challenges were all of the species V. maculifrons except in 1983, when V. flavopilosa was used. The frequency of sting autotomy noted for yellowjackets during field stings (4.5%) was significantly lower than the rate of 16.7% documented during the clinical sting challenges performed at the Mayo Clinic (P = 0.001) (Table 3). Although the field study yielded sting autotomy data for V. maculifrons slightly higher than yellowjackets as a group, the 7.1% rate is still lower than the 17.1% autotomy noted at the Mayo Clinic for V. maculifrons. The rates noted in the field study are sufficiently close so that the low number of observations does not demonstrate a significant difference. Although the rate of sting autotomy varied among years in the clinical sting challenges performed at the Mayo Clinic, the overall rate of 17.1% for V. maculifrons (omitting 1983 when V. flavopilosa was used) is significantly lower than the 64.6% reported by Greene et al. (1989) from challenges performed at the Johns Hopkins University, Baltimore, Md. (P < 0.001) (Table 4). Table 2. Yellowjacket sting autotomy rates during clinical sting challenges at the Mayo Clinic Year
No. stings
No. autotomies
% autotomy
1981 1982 1983° 1984 1985 1986 1987 1988 1989 Total, all years Total V. maculifrons only
22 15 31 28 34 42 43 46 45 306
2 2 4 12 7 14 7 1 2 51
9.1 13.3 12.9 42.9 20.6 33.3 16.3 2.2 4.4 16.7
275
47
17.1
"A nest of' V. flavopilosa was used as the source of insects for challenges performed in 1983.
March 1992
MULFINGER ET AL.: FREQUENCY OF STING AUTOTOMY
327
Table 3. Sting autotomy rates for field versus Mayo Clinic settings Source
No. stings °
All yellowjackets" Field Clinic V. viaculifrons only Field Clinic
No ..,.'. autotomies
% autotomy '
5 51
16.7
1 47
17.1
111
306 14
275
4.5
7.1
a
)C with Yates' correction = 9.4; P = 0.001.
Furthermore, when data collected in 1988 by Greene (Greene et al. 1989) are compared with data obtained during the same year at the Mayo Clinic, an even more pronounced difference in sting autotomy rate appears: 64.6 and 2.2%, respectively. Examination of scanning electron micrographs of vespid aculei revealed comparatively minor morphological differences among the various vespid species. Obvious differences were noted, however, between the sting morphologies of the vespid species and that of the honey bee. The micrographs revealed that the honey bee stylet is narrower than the width of the two barb-edged lancets, even when the lancets are completely retracted (Fig. 1). By contrast, vespid stylets were sufficiently wide to shield the serrated edges of the retracted lancets (Fig. 2). Additionally, the dorsal tips of A. mellifera stylets display between one and three rows of paired barbs (Fig. 3), whereas the entire dorsal side of all vespid Fig. 1—4. Retracted lancets of A. mellifera; only tip stylets are smooth (Fig. 4). of stylet visible in this ventral view. (2) Ventral view of Discussion When an aculeate hymenopteran stings under natural conditions, the initial insertion of the tip of the sting into the victim is accomplished by a quick movement at the end of the abdomen using the force of the body (Fig. 5). Subsequent deeper penetration is effected by the lancets, which alternate in back-and-forth boring movements while venom is being pumped and injected into the victim. At the same time, the stylet is being pulled deeply into the wound behind the lancets (Snodgrass 1956, Edwards 1980, Akre et al. 1981). This process is accomplished by rapid movements of the quadrate, triangular, and oblong plates, which in turn cause the lancets to move in extremely rapid succesTable 4. Sting autotomy rates for V. maculifrons during clinical sting challenges Source
No . stings
No. autotomies
% autotomya
Mayo Clinic Greene et al. (1989)
275 79
47 51
17.1 64.6
V = 69.1; P < 0.001.
V. maculifrons aculeus with stylet shielding serrated edges of lancets. (3) Dorsal tip of A. mellifera stylet with rows of barbs; lancet barbs exposed beyond stylet edges. (4) Smooth dorsal stylet tips of V. maculifrons concealing any indication of lancets on other side.
sion. This action is described by various workers as being a sawing movement, which literally cuts through skin fibers (Rietschel 1937, Hermann 1971, Edwards 1980, Akre et al. 1981). The phenomenon of sting autotomy has been studied extensively, and accordingly a number of explanations have been proposed. In a comparative study of the aculei of 102 species of Hymenoptera, Poore (1974) noted that the lancet barb size, type, and number for Apis and Vespula workers were not sufficiently different to account for sting autotomy in Apis. He suggested that a difference in muscular attachment between the sting and abdomen might be responsible. Snodgrass (1956) noted that in honey bees, the base of the aculeus has only a delicate membranous connection with the sting chamber walls and that only a gentle pull on the sting tip is necessary to extract the entire sting apparatus. The work of Rietschel (1937), which was confirmed by Maschwitz (1964), indicates that the
328
JOURNAL OF MEDICAL ENTOMOLOGY
Fig. 5. Sting sequence for Vespidae: (a) aculeus before insertion, (b) lancets pierce skin, (c) and (d) rapidly alternating lancets cut through fibers to penetrate deeper while venom is injected and the stylet is drawn into the wound, (e) lancets resheathed within the stylet, (f) and (g) aculeus withdraws from sting site, (h) ready to sting again, the entire process taking less than a second. This illustration has been adapted from a drawing by Albert Greene as Fig. 69 in Akre et al. (1981).
Vol. 29, no. 2
ing the stinging process, it is able to avoid resistance during withdrawal by reshielding the lancet barbs within the stylet. The discrepancies in autotomy rates noted during clinical sting challenges, both the year-toyear variations within the Mayo Clinic data and the large difference between the Mayo Clinic and Greene et al. (1989) data, suggest that the frequency of sting autotomy during clinical sting challenges is dependent upon the sting delivery and withdrawal methods used. The findings of this study also indicate that the autotomy rate of 64.6% for V. maculifrons reported by Greene et al. (1989) is not representative of the autotomy rate for this insect under natural conditions. Acknowledgment This work was supported in part by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University, manuscript 180-91. Publication costs were supported by Vespa Laboratories, Inc., Spring Mills, Pa. References Cited
spiracular plate muscles are especially reduced in Apis workers and that this constitutes a preformed breaking point for true autotomy to occur. By contrast, in vespids the spiracular plate is rigidly connected in combination with strong muscles. This difference grants vespids a powerful and maneuverable aculeus, whereas a honey bee can move its sting only forward and backward, with the resultant anchorage of the sting being stronger than its breaking strength (Maschwitz & Kloft 1971). The differences noted in electron micrographs of the examined hymenopteran aculei provide a further explanation for the different rate in sting autotomy noted between honey bees and social wasps. Barbs on the dorsal tip of the honey bee stylet have also been reported by Weiss (1978). These structures, which are absent on the smooth vespid stylet, together with the observation that the entire edge of the honey bee lancet extends over any possible protective edge of the stylet, can additionally account for the increased anchorage of the honey bee sting. Morphological differences also help provide an explanation for the different autotomy rates noted for V. maculifrons by Greene et al. (1989), the Mayo Clinic, and the field sting survey. During clinical sting challenges, it is likely that the insect is pulled from the site before normal sting action is complete. If the yellowjacket is forcibly removed from the sting site before the lancets are resheathed by the stylet, the lancet barbs would still be well anchored in the skin. Conversely, if the yellowjacket is uninterrupted dur-
Akre, R. D., A. Greene, J. F. MacDonald, P. J. Landolt & H. G. Davis. 1981. Yellowjackets of America north of Mexico. U.S. Dep. Agric. Handb. 552. Edwards, R. 1980. Social wasps. Rentokil, E. Grinstead, England. Greene, A., N. L. Breisch, D.B.K. Golden, K. A. Kwiterovich, B. I. Addison, K. C. Schuberth & L. M. Lichtenstein. 1989. The sting that stays: autotomy in two common yellowjacket species. J. Allergy Clin. Immunol. 83(1): 229. Hermann, H. R. 1971. Sting autotomy, a defensive mechanism in certain social Hymenoptera. Insectes Soc. 18: 111-120. Liepens, A. & E. DeHaven. 1978. A rapid method of cell drying for scanning electron microscopy. Scanning Electron Microsc. 2: 37-44. Maschwitz, U. 1964. Gefahrenalarmstoffe und Gefahrenalarmierung bei sozialen Hymenopteran. Z. Vgl. Physiol. 47: 596-655. Maschwitz, U. & W. Kloft. 1971. Morphology and function of the venom apparatus of insects—bees, wasps, ants, and caterpillars, pp. 1-60. In W. Bucherl & E. E. Buckley [eds.], Venomous animals and their venom, vol. 3. Academic, New York. Poore, D. M. 1974. Comparative study of the lancets and sheaths of some aculeate Hymenoptera. Bull. Southern Calif. Acad. Sci. 73: 42-47. Rietschel, P. 1937. Bau und Funktion des Wehrstachels der staatenbildenden Bienen und Wespen. Z. Morphol. Okol. Tiere 33: 313-357. Snodgrass, R. E. 1956. Anatomy of the honey bee. Comstock, Ithaca, N.Y. Weiss, J. 1978. Comparative morphology of the sting apparatus in the genus Apis (Hymenoptera: Apidae). Apidologie 9: 19-32. Received for publication 11 July 1991; accepted 9 September 1991.
J. Med. Entomol 29(2): 325-328 (1992)
ABSTRACT Data from clinical sting challenge investigations indicate that certain species of yellowjackets experience sting apparatus autotomy with surprisingly high frequency. However, a retrospective survey of vespid collectors strongly supports the supposition that the frequency of yellowjacket sting autotomy seen in clinical situations is not representative of the frequency experienced under field conditions. Examination by electron microscopy of the sting apparatuses of several vespid species and that of Apis mellifera L., the honey bee, revealed previously unreported structural variations between apid and vespid aculei which likely contribute to differences in sting autotomy rates observed between the honey bee and the social wasps. Specifically, when the lancets of a vespid aculeus are in a retracted position, the width of the smooth-edged stylet extends beyond the barbed edges of the lancets, forming a protective sheath. By contrast, all honey bee aculei possess stylets of insufficient width to shroud the barbs of retracted lancets, thus allowing the barbs to be completely exposed. Additionally, the dorsal surface of all vespid stylets are smooth in contrast to the dorsal surface of honey bee stylets, which support from one to three rows of paired barbs. The exposure of barbs on retracted honey bee lancets and the presence of additional barbs on the dorsal tip of the stylet would make withdrawal of a honey bee aculeus from a victim's flesh more difficult than withdrawal of a vespid aculeus, in which the barbs of retracted lancets are shielded and no dorsal barbs are present. KEY WORDS Insecta, Hymenoptera, sting autotomy, sting morphology
I T HAS LONG BEEN ACCEPTED among aculeate hymenopterists, as well as allergists trained in insect sting hypersensitivity, that the sting autotomy (self-amputation of sting apparatus) rate of the honey bee, Apis mellifera L., approaches 100%, whereas the sting autotomy rate of other medically important hymenopterans, principally of the family Vespidae, is very low. However, a recent report cites an unusually high frequency of sting autotomy with the eastern yellowjacket, Vespula maculifrons (Buysson). During clinical sting challenges performed to evaluate the efficacy of hymenopteran venom immunotherapy, 51 of 79 V. maculifrons workers reportedly experienced sting autotomy (Greene et al. 1989). A number of field-experienced Hymenoptera r i.•ii.r J i.u u • u r specialists round the high frequency of sting autotomy noted by Greene etal. (1989) to be inconsistent with personal experiences. Few had ever experienced stings by yellowjackets that re-
suited in autotomy of the sting apparatus. In an effort to clarify the apparent discrepancy between the frequencies of sting autotomy observed during clinical sting challenges and field sting incidents, the frequencies of sting autotomy for honey bees; yellowjackets, Vespula spp.; aerial yellowjackets, Dolichovespula arenaria (F.); baldfaced hornets, Dolichovespula maculata (L.); and paper wasps, Polistes spp. were evaluated in both field and clinical situations. In addition, aculei from A. mellifera and five different vespid species were examined by scanning electron microscopy (SEM) to determine what external morphological differences exist, A»
. . i j X/f . u i Materials and Methods
The frequencies of sting autotomy among honey bees, yellowjackets, aerial yellowjackets, baldfaced hornets, and paper wasps under "field" conditions were evaluated by means of a questionnaire addressed to 50 experienced 1 Research Laboratory for Allergic Diseases, The Mayo j questionnaire was a retroc i( c o l l e c t o r s > T h e
Clinic, Rochester, Minn. 55905.
.
2 Ohio Agricultural Research and Development Center, Ohio State University, Wooster, Ohio 44691.
i • i
i
j j . -i
r J_I
spective Survey which asked details ot the respondent's last five sting encounters with the
0022-2585/92/0325-0328$02.00/0 © 1992 Entomological Society of America
326
Vol. 29, no. 2
JOURNAL OF MEDICAL ENTOMOLOGY
aforementioned species. The details included the anatomical site of each sting, whether the sting was inflicted on bare skin versus through clothing or protective netting, the identity of the hymenopteran (honey bee, yellowjacket, aerial yellowjacket, bald-faced hornet, or paper wasp), whether the sting was interrupted by swatting or brushing away the insect, and whether the insect experienced sting autotomy at the site. The collectors were also asked to identify, if possible, the species of Vespula when the insect belonged to that genus. The frequencies of sting autotomy during clinical hymenopteran sting challenges were assessed by the Research Laboratory for Allergic Diseases of the Mayo Clinic, Rochester, Minn. Records were kept on the frequencies of autotomy among various hymenopteran species during sting challenges performed between 1980 and 1989. V. maculifrons was the only yellowjacket species tested in clinical sting challenges except in 1983, when several challenges were performed with V. flavopilosa Jacobson. Challenges were executed using a pair of forceps to hold an insect in contact with the upper arm of a human subject until a sting was inflicted. The insect was removed as soon as the subject felt a sting. Insects prepared for SEM were supplied and identified by Vespa Laboratories, Inc. (Spring Mills, Pa.). The insects were quick-frozen upon collection and stored at — 20°C to avoid deterioration. The sting apparatus from thawed specimens was carefully dissected, placed in 70% ethanol, then dehydrated using an ethanol-freon TF dehydration procedure (Liepens & DeHaven 1978). Once dried, the aculei were attached to specimen stubs using low-resistance silver paint (E. Fullam, Latham, N.Y.) and then sputtercoated with approximately 15 nm of platinum using an E5100 series Polaron coater. Specimens were viewed and photographed with an ISI40 scanning electron microscope at magnifications ranging from 20 to 5,000 x. A minimum of five insects of each species was examined. Results The frequency of sting autotomy under field sting conditions for each group of the medically important Hymenoptera was established from the survey of collectors (Table 1). The tabulation included only those stings delivered directly into unclothed skin and without interruption by swatting or brushing away the insect. The sting autotomy rates noted for yellowjackets, aerial yellowjackets, baldfaced hornets, and paper wasps were all low, and there was no suggestion that the rate for any one of these groups was significantly higher than the others. However, as expected, the autotomy rate for honey bees was
Table 1. Sting autotomy rates during uninterrupted field stings Insect
No. stings
No. autotomies
% autotomy
Vespula spp. (incl. V. maculifrons) V. maculifrons only D. arenaria D. maculata Polistes spp. A. mellifera
111 14 12 20 33 89
5 1 1 1 2 71
4.5 7.1 8.3 5.0 6.1 79.8"
"X1 with Yates' correction = 149.4; P < 0.001.
significantly higher than the rates noted for the social wasps (P < 0.001). The frequency of yellowjacket sting autotomy during the past 9 yr of clinical sting challenges performed at the Mayo Clinic was tabulated from clinic records on a yearly basis (Table 2). Yellowjackets used in these sting challenges were all of the species V. maculifrons except in 1983, when V. flavopilosa was used. The frequency of sting autotomy noted for yellowjackets during field stings (4.5%) was significantly lower than the rate of 16.7% documented during the clinical sting challenges performed at the Mayo Clinic (P = 0.001) (Table 3). Although the field study yielded sting autotomy data for V. maculifrons slightly higher than yellowjackets as a group, the 7.1% rate is still lower than the 17.1% autotomy noted at the Mayo Clinic for V. maculifrons. The rates noted in the field study are sufficiently close so that the low number of observations does not demonstrate a significant difference. Although the rate of sting autotomy varied among years in the clinical sting challenges performed at the Mayo Clinic, the overall rate of 17.1% for V. maculifrons (omitting 1983 when V. flavopilosa was used) is significantly lower than the 64.6% reported by Greene et al. (1989) from challenges performed at the Johns Hopkins University, Baltimore, Md. (P < 0.001) (Table 4). Table 2. Yellowjacket sting autotomy rates during clinical sting challenges at the Mayo Clinic Year
No. stings
No. autotomies
% autotomy
1981 1982 1983° 1984 1985 1986 1987 1988 1989 Total, all years Total V. maculifrons only
22 15 31 28 34 42 43 46 45 306
2 2 4 12 7 14 7 1 2 51
9.1 13.3 12.9 42.9 20.6 33.3 16.3 2.2 4.4 16.7
275
47
17.1
"A nest of' V. flavopilosa was used as the source of insects for challenges performed in 1983.
March 1992
MULFINGER ET AL.: FREQUENCY OF STING AUTOTOMY
327
Table 3. Sting autotomy rates for field versus Mayo Clinic settings Source
No. stings °
All yellowjackets" Field Clinic V. viaculifrons only Field Clinic
No ..,.'. autotomies
% autotomy '
5 51
16.7
1 47
17.1
111
306 14
275
4.5
7.1
a
)C with Yates' correction = 9.4; P = 0.001.
Furthermore, when data collected in 1988 by Greene (Greene et al. 1989) are compared with data obtained during the same year at the Mayo Clinic, an even more pronounced difference in sting autotomy rate appears: 64.6 and 2.2%, respectively. Examination of scanning electron micrographs of vespid aculei revealed comparatively minor morphological differences among the various vespid species. Obvious differences were noted, however, between the sting morphologies of the vespid species and that of the honey bee. The micrographs revealed that the honey bee stylet is narrower than the width of the two barb-edged lancets, even when the lancets are completely retracted (Fig. 1). By contrast, vespid stylets were sufficiently wide to shield the serrated edges of the retracted lancets (Fig. 2). Additionally, the dorsal tips of A. mellifera stylets display between one and three rows of paired barbs (Fig. 3), whereas the entire dorsal side of all vespid Fig. 1—4. Retracted lancets of A. mellifera; only tip stylets are smooth (Fig. 4). of stylet visible in this ventral view. (2) Ventral view of Discussion When an aculeate hymenopteran stings under natural conditions, the initial insertion of the tip of the sting into the victim is accomplished by a quick movement at the end of the abdomen using the force of the body (Fig. 5). Subsequent deeper penetration is effected by the lancets, which alternate in back-and-forth boring movements while venom is being pumped and injected into the victim. At the same time, the stylet is being pulled deeply into the wound behind the lancets (Snodgrass 1956, Edwards 1980, Akre et al. 1981). This process is accomplished by rapid movements of the quadrate, triangular, and oblong plates, which in turn cause the lancets to move in extremely rapid succesTable 4. Sting autotomy rates for V. maculifrons during clinical sting challenges Source
No . stings
No. autotomies
% autotomya
Mayo Clinic Greene et al. (1989)
275 79
47 51
17.1 64.6
V = 69.1; P < 0.001.
V. maculifrons aculeus with stylet shielding serrated edges of lancets. (3) Dorsal tip of A. mellifera stylet with rows of barbs; lancet barbs exposed beyond stylet edges. (4) Smooth dorsal stylet tips of V. maculifrons concealing any indication of lancets on other side.
sion. This action is described by various workers as being a sawing movement, which literally cuts through skin fibers (Rietschel 1937, Hermann 1971, Edwards 1980, Akre et al. 1981). The phenomenon of sting autotomy has been studied extensively, and accordingly a number of explanations have been proposed. In a comparative study of the aculei of 102 species of Hymenoptera, Poore (1974) noted that the lancet barb size, type, and number for Apis and Vespula workers were not sufficiently different to account for sting autotomy in Apis. He suggested that a difference in muscular attachment between the sting and abdomen might be responsible. Snodgrass (1956) noted that in honey bees, the base of the aculeus has only a delicate membranous connection with the sting chamber walls and that only a gentle pull on the sting tip is necessary to extract the entire sting apparatus. The work of Rietschel (1937), which was confirmed by Maschwitz (1964), indicates that the
328
JOURNAL OF MEDICAL ENTOMOLOGY
Fig. 5. Sting sequence for Vespidae: (a) aculeus before insertion, (b) lancets pierce skin, (c) and (d) rapidly alternating lancets cut through fibers to penetrate deeper while venom is injected and the stylet is drawn into the wound, (e) lancets resheathed within the stylet, (f) and (g) aculeus withdraws from sting site, (h) ready to sting again, the entire process taking less than a second. This illustration has been adapted from a drawing by Albert Greene as Fig. 69 in Akre et al. (1981).
Vol. 29, no. 2
ing the stinging process, it is able to avoid resistance during withdrawal by reshielding the lancet barbs within the stylet. The discrepancies in autotomy rates noted during clinical sting challenges, both the year-toyear variations within the Mayo Clinic data and the large difference between the Mayo Clinic and Greene et al. (1989) data, suggest that the frequency of sting autotomy during clinical sting challenges is dependent upon the sting delivery and withdrawal methods used. The findings of this study also indicate that the autotomy rate of 64.6% for V. maculifrons reported by Greene et al. (1989) is not representative of the autotomy rate for this insect under natural conditions. Acknowledgment This work was supported in part by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University, manuscript 180-91. Publication costs were supported by Vespa Laboratories, Inc., Spring Mills, Pa. References Cited
spiracular plate muscles are especially reduced in Apis workers and that this constitutes a preformed breaking point for true autotomy to occur. By contrast, in vespids the spiracular plate is rigidly connected in combination with strong muscles. This difference grants vespids a powerful and maneuverable aculeus, whereas a honey bee can move its sting only forward and backward, with the resultant anchorage of the sting being stronger than its breaking strength (Maschwitz & Kloft 1971). The differences noted in electron micrographs of the examined hymenopteran aculei provide a further explanation for the different rate in sting autotomy noted between honey bees and social wasps. Barbs on the dorsal tip of the honey bee stylet have also been reported by Weiss (1978). These structures, which are absent on the smooth vespid stylet, together with the observation that the entire edge of the honey bee lancet extends over any possible protective edge of the stylet, can additionally account for the increased anchorage of the honey bee sting. Morphological differences also help provide an explanation for the different autotomy rates noted for V. maculifrons by Greene et al. (1989), the Mayo Clinic, and the field sting survey. During clinical sting challenges, it is likely that the insect is pulled from the site before normal sting action is complete. If the yellowjacket is forcibly removed from the sting site before the lancets are resheathed by the stylet, the lancet barbs would still be well anchored in the skin. Conversely, if the yellowjacket is uninterrupted dur-
Akre, R. D., A. Greene, J. F. MacDonald, P. J. Landolt & H. G. Davis. 1981. Yellowjackets of America north of Mexico. U.S. Dep. Agric. Handb. 552. Edwards, R. 1980. Social wasps. Rentokil, E. Grinstead, England. Greene, A., N. L. Breisch, D.B.K. Golden, K. A. Kwiterovich, B. I. Addison, K. C. Schuberth & L. M. Lichtenstein. 1989. The sting that stays: autotomy in two common yellowjacket species. J. Allergy Clin. Immunol. 83(1): 229. Hermann, H. R. 1971. Sting autotomy, a defensive mechanism in certain social Hymenoptera. Insectes Soc. 18: 111-120. Liepens, A. & E. DeHaven. 1978. A rapid method of cell drying for scanning electron microscopy. Scanning Electron Microsc. 2: 37-44. Maschwitz, U. 1964. Gefahrenalarmstoffe und Gefahrenalarmierung bei sozialen Hymenopteran. Z. Vgl. Physiol. 47: 596-655. Maschwitz, U. & W. Kloft. 1971. Morphology and function of the venom apparatus of insects—bees, wasps, ants, and caterpillars, pp. 1-60. In W. Bucherl & E. E. Buckley [eds.], Venomous animals and their venom, vol. 3. Academic, New York. Poore, D. M. 1974. Comparative study of the lancets and sheaths of some aculeate Hymenoptera. Bull. Southern Calif. Acad. Sci. 73: 42-47. Rietschel, P. 1937. Bau und Funktion des Wehrstachels der staatenbildenden Bienen und Wespen. Z. Morphol. Okol. Tiere 33: 313-357. Snodgrass, R. E. 1956. Anatomy of the honey bee. Comstock, Ithaca, N.Y. Weiss, J. 1978. Comparative morphology of the sting apparatus in the genus Apis (Hymenoptera: Apidae). Apidologie 9: 19-32. Received for publication 11 July 1991; accepted 9 September 1991.
