5^2 THE INVERTEBRATA 



In ichneumons, chalcids and cynipids the ovipositor issues far for- 

 wards beneath the abdomen, and these insects differ in this feature from 

 the Proctotrypidae in which the ovipositor is terminal. Dipterous larvae 

 are often parasitized by these insects, as are also the eggs of Orthop- 

 tera and Hemiptera. Many hyper-parasites, i.e. parasites of other 

 parasites, occur in this family. Phaenoserphus is parasitic on carabid 

 beetle larvae (Fig. 345), and Inostemma is an egg-parasite of dipterous 

 gall midges. 



Whereas parasitism is a character, largely though not wholly, 

 common to the foregoing families, the ants, wasps and bees next to 

 be considered show a tendency, in varying degrees, towards the 

 development of the social habit. 



The ants (Formicotdea) are social, polymorphic insects in which 

 two segments are involved in the formation of the abdominal petiole. 

 Further, this petiole is always characterized by the possession of one 

 or two nodes (Fig. 346 E). The females are endowed with a well- 

 developed sting, the modified ovipositor. Polymorphism reaches its 

 highest degree of complexity in this group, as many as twenty-nine 

 morphologically different castes having been recognized. Some of 

 these are pathological phases due to infection by parasites, e.g. 

 Nematode worms or other Hymenoptera. In such colonies as produce 

 winged forms of both sexes, mating takes place during a nuptial flight 

 in which several colonies in one neighbourhood indulge at the same 

 time. This ensures intercrossing between individuals from different 

 colonies. The females then cast off their wings and start colonies in 

 the ground, each one for itself. The workers are sterile females, whose 

 power to lay eggs in certain circumstances may return. For instance, 

 when a colony loses a queen several workers may, under the influence 

 of suitable diet, take her place. In addition to the environmental 

 complexity which a social existence involves, the lives of ants are 

 further complicated by association with other organisms. Some, e.g. 

 certain myrmecine ants, have adopted an agricultural habit, living on 

 fungi which they specially cultivate. Others gather seeds from which 

 they destroy the radicle to prevent germination, special chambers or 

 granaries in the nest being constructed for their storage. The pastoral 

 habit characterizes others, a symbiotic relation being set up with such 

 insects, e.g. Aphides, as exude fluids which are palatable to the ants. 

 In addition to associations of this kind there are numerous others of 

 an indifferent or little understood nature, but which may range from 

 the symbiotic to the parasitic. Finally may be mentioned the slave- 

 makers; Formica sanguinea, for instance, captures from the colonies 

 of F. fusca, pupae which on emergence serve as slaves in the colony 

 which has adopted them. 



The wasps of the super-family Vespoidea are both social and 



