CHAPTER 12 



Aquatic Hymenoptera 



By K. S. Hagen 



University of California, Albany 



All the Hymenoptera known to be aquatic are appar- 

 ently entomophagous. The superfamilies Ichneumo- 

 noidea, Chalcidoidea, Proctotrupoidea, and the family 

 Pompilidae include species in which the adults have 

 become adapted to penetrating water surfaces. This 

 adaptation was necessitated by the occurrence beneath 

 the water surface of the host in which they develop. 

 A pompilid species is an exception in that it enters 

 the water to obtain its prey which it then transports 

 from the water to its terrestrial larval cell. 



Respiration. — Since larvae of endoparasitic Hymen- 

 optera live surrounded by animal tissues, the micro- 

 habitats of aquatic species are quite similar to those 

 of species which infest terrestrial hosts. It is not 

 surprising, therefore, that parasitic larvae of terres- 

 trial and aquatic Hymenoptera do not differ greatly 

 in their adaptations, and that some genera (including 

 both Trichopria and Dacnusa) contain both terrestrial 

 and aquatic species (Berg, 1949). 



Among ectoparasitic insects an outstanding example 

 of a distinct respiratory adaptation in the immature 

 stages is found in the ichnoumonoid family Agriotypi- 

 dae. The two known species of this family are 

 parasitic upon the prepupae and pupae of caddisflies 

 in their cases. Agriotypus armatus Walker of Europe 

 (Fisher, 1932) and A. gracilis Wtstn. of Japan (Clausen, 

 1931) attach their eggs to the host body, and the lar- 

 vae feed externally throughout their period of activity. 

 Although they are immersed in water, the early larval 

 instars have no morphological adaptations for res- 

 piration, their oxygen requirements evidently being 

 fulfilled by diffusion from the water that flows through 

 the case (Clausen, 1950). 



Clausen (1950) goes on to say that "as soon as 

 the host dies, which is during the last larval stage 

 of the parasite, the flow of water through the case 

 ceases, and the parasite larva must obtain an adequate 

 air supply in some other way. Various early observers 

 had noted that the caddisfly cases containing mature 

 larvae, pupae, or adults of A. armatus always pos- 

 sessed, at the anterior end, a silken ribbon about 

 one millimeter in width and several centimeters in 

 length, which was free-floating in the surrounding 



water. W. Muller (1889, 1891) determined that death of 

 the parasite usually followed the removal of the rib- 

 bon, and Henriksen (1922) vorified this observation 

 and was first to ascribe a respiratory function to it. 

 The ribbon of A. gracilis not only serves the respir- 

 atory needs of the full grown larva and pupa but also 

 the adult through its entire winter hibernation period. 



"The adult female, which must enter the water for 

 oviposition, is clothed with a dense pubescence that 

 holds an air bubble over the entire body while she is 

 submerged. Both A. armatus and A. gracilis need to 

 emerge at intervals of about ten to fifteen minutes to 

 renew their air supply. The oxygen content of the 

 enveloping bubble is doubtless appreciably supple- 

 mented from the surrounding water" (Clausen, 1950)% 



Survey of aquatic species. — Many hymenopterous 

 species that parasitize aquatic insects will not be 

 treated in this discussion. These species are omitted 

 because they do not attack their hosts beneath the 

 water surface, but oviposit in or on a host that is not 

 submerged. 



In California there are only two species of Hymen- 

 optera recorded as being aquatic. One is a mymarid 

 and the other a braconid. However, as soon as a 

 special search is made, numerous species of aquatic 

 Hymenoptera undoubtedly will be discovered. 



The following key treats the recognized Nearctic 

 genera which include species that are either known 

 or suspected of entering the water in the adult stage. 

 The discussion is of necessity based largely on 

 published observations by investigators in the eastern 

 United States. 



A Generic Key to the Adults of 

 Nearctic Aquatic Hymenoptera 



1. Pronotum extending back to tegulae 2 



— Pronotum not extending back to tegulae 



CHALCIDOIDEA ... 10 



2. Trochanters 1 -segmented 3 



— Trochanters 2-segmented . . ICIINEUMONOIDEA ... 5 



3. Wing venation well developed, hind wing with several 

 veins, and at least 1 closed cell; large species ..... 



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