Little Bags of Horror: Three Obscure Families of Surreal Aculeates

Ah, the love of parasites that runs through our species--the grim fascination with the cloud of bacteria, protists, fungi, and animals that depend upon Homo sapiens for sustenance and shelter. The disgusted fixation on the worms that haunt our gut and veins, emitting eggs to be sent forth to other hosts via our, ah, "dark materials"...or maybe this is all just my personal fascination: mine, and that of the other members of this perverted cult.

Perhaps this fascination originates in our ontogeny: we, like all viviparous creatures, play host for our gestating, "parasitic" fetuses; and they suck away our vitality and alter our behavior (i.e., cause us to host baby showers and purchase adorably tiny shoes) for nine long months. ("Us" being mothers, of course. And, no, I am not a mother, or even a female. But anyway...) This grotesque mystique is what engenders such works as (for example) the "Alien" franchise, and ensures their continued popularity. 

Of course, any biologist worth his snuff knows that the eponymous extraterrestrial of the said horror franchise (fig. 1; drawn by H. R. Giger) is unequivocally not a parasite. It kills its host (in grisly fashion)--not by occasional accident, but by necessity. Therefore, it is a parasitoid: and parasitoids are infinitely more disturbing than true parasites.And insects (my particular area of expertise) harbor a wide range of parasitoids. Indeed, the 60,000 species of ichneumon wasps (Ichneumonidae), and the remainder of the panoply of parasitoid wasps, are perhaps the best-known exemplars of the lifestyle: a lifestyle whose prevalence was cited explicitly by Charles Darwin as evidence against a benevolent Creator (Gould, 1994). But I wouldn't post about something well-known, now would I? No; instead I will discuss some parasitoid wasp families that I think are sadly obscure.These families are included within the Aculeata--the clade of the order Hymenoptera consisting of those wasps (to use that term in a phylogenetic sense) that possess a stinger (a diagram of one can be found at right). As the organ is a specialized ovipositor, only females bear one (although a good deal of lineages have secondarily lost theirs). A stinger is a versatile instrument: one can use it for self-defense, to kill prey, subdue a rival, or even as an impromptu shish-kebab for carrying said prey back to one's nest (as Oxybelus sp. do; Peckham, 1985). Consequently, aculeates run a gamut of lifestyles: some are parasitoids that use their stingers to stun their offsprings' hosts; others use theirs to paralyze prey that will be hauled back to a burrow, where the individual will be devoured by the wasp's larvae; and still others use it in the service of their colony.Three superfamilies comprise the Aculeata: Chrysidoidea, Vespoidea, and Apoidea; all include some parasitoids. Interestingly, all aculeates who grow in that manner are ectoparasitoids: that is, they live and grow as larvae on their hosts' bodily exteriors (in contrast to most hymenopteran parasitoids, who dine from the interior). All of them, that is, except the members of two related families: the Dryinidae and Embolemidae.

Dryinids are odd to look upon, as one can see from the photograph by Stephen Luk at right of Dryinus alatus; their physique is fairly typical of a hymenopteran, but their demeanor has been compared to that of a long-legged fly (Micropezidae), and the most distinctive attribute of the Dryinidae (found only in the females) seems to have been robbed from some crustacean. I refer here to the wasps' chelae, or pincers. Females of most species (1,100 worldwide; van Noort, 2012a) possess them, with the exceptions belonging to the subfamilies Aphelopinae and Diaphelopinae (O'Neill, 2001). The pincers derive from a modification of the apical segment of the foreleg's tarsus--lengthened and tilted into a blade--and one of the tarsal claws, elongated and enlarged as a counterpoint to the aforementioned tarsal segment. The other tarsal claw (all insects have two) is either vestigial or absent (Olmi, 1984). (See the chela of an Anteon urbani at left.) The inner surfaces of the chela's two pinching scimitars may bear blunt lamellae, hairs, or some combination thereof to increase grip; female dryinids utilize their pincers to restrain the unfortunates who will be hosts for their larvae: leafhoppers (Cicadellidae) or planthoppers (usually Flatidae or Delphacidae).Since 12 families of bugs (all in the hemipteran suborder Auchenorrhyncha) are represented in the range of dryinid hosts (van Noort, 2012), the precise chelate design of the Dryinidae varies: ranging from the unadorned, robust ones of the Bocchinae to the diminutive pincers of the Gonatopodinae to the surreal sawtoothed futuristic-instruments-of-dental-torture (or something like those) borne by Megadryinus magnificus (Dryininae)--sadly, I was unable to locate a digital picture of these strangely narrow, rather fragile-appearing chelae (with 150 parallel rows of lamellae opposite a hundred pointed hairs), which account for 40% of the wasp's total length (O'Neill, 2001).But the most noteworthy peculiarity of the Dryinidae lies in their larvae, which, as I mentioned before, reside within--rather than without--their host, in contrast to the remainder of the Aculeata, which are ectoparasitoids almost as a rule. This difference in dryinid ontogeny makes an appearance even before hatching, since the mother lays the lone egg inside a host (inserting her ovipositor through a gap in the integument). After a brief period, though (Clausen, 1940), a dryinid larva's lifestyle shifts from that of a pure internal parasitoid to one lying somewhere between that state and that of an external one: its head remains embedded in the host, but the body hangs outside, protected from the elements by a thylacium--a bubble-shaped sack comprised of accumulated larval exoskeletons. Despite the fact that this encumbrance is hardly concealed (see photograph by Claude Pilon at right), the host goes on its merry way as if nothing were amiss; at least, that is, until the time comes for the larva's pupation: then the dryinid ruptures its thylacium and devours the host in plain sight. What a rude awakening awaits the planthopper nymph (Fulgoridae) above. With the exception of the aphelopine Crovettia theliae (an endoparasitoid for the duration of the larval stage, and incidentally the only aculeate to exhibit polyembryony*; Kornhauser, 1919), all Dryinidae create a thylacium. So does the single member of the closely related (Carpenter, 1986) Embolemidae whose habits are known. In contrast to the diverse Dryinidae, only twenty species of embolemid are described (van Noort, 2012b), and they are seldom collected. Sexual dimorphism is strong, with the fleet-footed females conspicuously lacking in wings (Watson & Dallwitz, 2003): see the specimen of Embolemus nearcticus collected by Jeff Gruber above. (A good number of female dryinids are apterous as well.) These females are soil-dwelling; they have been noted as residing in ant nests (Donisthorpe, 1927) and mammal burrows (Heim de Balsac, 1935), but why they do so is anyone's guess, since the only known embolemid hosts (belonging to Ampulicomorpha confusa) are planthopper nymphs (Achilidae; Birdwell, 1958), which are not noted for a fossorial lifestyle.These two singular families, Dryinidae and Embolemidae, are members of the Chrysidoidea, a good-sized superfamily mostly comprised of small, obscure taxa (7 in all): of these only the brilliantly metallic cuckoo wasps (Chrysidinae), named for their brood parasitism of assorted solitary bees and wasps, are familiar to the general public. This is not the case with another previously-mentioned aculeate superfamily--the Vespoidea, which includes all social wasps (in three subfamilies of Vespidae), potter wasps (Vespidae: Eumeninae), the notorious tarantula hawk wasps (Pepsis sp., Pompilidae), the velvet ants (Mutillidae), and the ubiquitous ants (Formicidae), to name but the famous ones. Of course, a good share of vespoid families (10 living) are just as little-known as the average chrysidoid taxon. Among these is the last family of parasitoid wasp to be discussed in this post: the Rhopalosomatidae.Rhopalosomatids (38 spp.) are rare ectoparasitoid wasps that in most genera (as adults) strongly resemble ichneumonids: the eastern North American Rhopalosoma nearcticum (above, photographed by Rich Hoyer) bears a strikingly ophionine mien. Those in the genus Olixon, however, are brachypterous, with rudimentary flaps for wings (see the holotype of O. toliaraensis at left) and could be easily mistaken for velvet ants: in fact, the genus was initially classified in the Ichneumonidae (presumably on the basis of a seeming resemblance to certain members of the Phygadeuontini, a tribe in that family), and placed variously in the Pompilidae, Bethylidae, Braconidae, and none other than the Dryinidae throughout the late nineteenth and early twentieth centuries (Guidotti, 1999). Olixon is also unique among flightless hymenopterans in that (insofar as I can tell) both genders are brachypterous.Rhopalosomatids warrant description in this post because of their larvae, which are protected by sacs of accumulated exuviae strongly reminiscent of the aforementioned chrysidoid, half-endoparasitoid families' thylacia (Gurney, 1953). Despite this resemblance, the young of Rhopalosomatidae are just as much ectoparasitoids as other Vespoidea. Why, then, the quasi-thylacium? Probably the shelter is necessary because rhopalosomatid hosts are exclusively crickets (Gryllidae), which remain very much active even during their affliction (see photograph by Roy Sigafus at right): by contrast, most vespoid parasitoids feed on insects with more recumbent habits, or ones that at least dwell in unexposed environs (e.g., the ever-popular subterranean scarab beetle grubs). This feature--"little bags of horror"--is what makes the Rhopalosomatidae so peculiar (and the other two families discussed herein as well).Well, then: now that I have poured further carbon dioxide into our already overloaded atmosphere via the usage of this computer, I will bid farewell to any readers who may peruse this inaugural post of Life, et al.   *To put it succinctly, polyembryony is a state with which OctoMom is all too familiar. _______________________________________________________________ Birdwell, J. C. (1958). Biological notes on Ampulicomorpha confusa Ashmead and its fulgoroid host. Proceedings of the Entomological Society of Washington, 60, 23-26. Carpenter, J. M. (1986). Cladistics of the Chrysidoidea. Journal of the New York Entomological Society, 94, 303-330. Clausen, C. P. (1940). Entomophagous Insects. New York City: McGraw-Hill. Donisthorpe, H. St. J. K. (1927). The Guests of British ants: their Habits and Life-Histories. London: G. Routledge & Sons. Heim de Balsac, H. (1935). Ecologie de Pedinomma rufescens Westwood; sa presence dans les nids des micromammiferes (Hym. Embolemidae). Revue Francaise d'Entomologique, 2, 109-112. Gould, S. J. (1994). Hen's Teeth and Horse's Toes: Further Reflections in Natural History. New York City: W. W. Norton. Guidotti, A. E. (1999). Systematics of Little-Known Parasitic Wasps of the Family Rhopalosomatidae (Hymenoptera: Vespoidea) [electronic version]. University of Toronto (unpublished). Retrieved 12/9/12 from https://tspace.library.utoronto.ca/bitstream/1807/14562/1/MQ46189.pdf Gurney, A. B. (1953). Notes on the biology and immature stages of a cricket parasite of the genus Rhopalosoma. Proceedings of the United States National Museum, 103, 19-34.Kornhauser, S. I. (1919). The sexual characteristics of the membracid, Thelia bimaculata (Fabr.) J. Morphol., 32, 531-636.   Olmi, M. (1984). Revision of the Dryinidae. Memoirs of the American Entomological Institute, 37, 1-1913.O'Neill, K. M. (2001). Solitary Wasps: Behavior and Natural History. Ithaca: Cornell University Press.   Peckham, D. J. (1985). Ethological observations on Oxybelus (Hymenoptera: Sphecidae) in southwestern New Mexico. Annals of the Entomological Society of America, 78, 865-872.Van Noort, S. (2012a). Dryinidae: dryinid wasps of Africa and Madagascar. Retrieved 12/9/12 from http://www.waspweb.org/Chrysidoidea/Dryinidae/index.htm Van Noort, S. (2012b). Embolemidae: embolemid wasps of Africa and Madagascar. Retrieved 12/9/12 from http://www.waspweb.org/Chrysidoidea/Embolemidae/index.htmWatson, L. & Dallwitz, M. J. (2003). British Insects: the Families of Hymenoptera. Retrieved 12/7/12 from http://delta-intkey.com/britin/hym/www/embolemi.htm