760 



CLASS IV. INSECTA. 



flies Some again are dimorphic : the female Scleroderma is usually apter- 

 ous ; when wings are present ocelli always appear, though they are as a rule 

 absent in wingless forms. Platygaster (Fig. 490) infests the larvae of Cecido- 

 myia and helps to diminish the numbers of the Hessian-fly. Polyembryony 

 exists in this and in the following family (v. p. 641). Polynema natans is a 

 remarkable aquatic form which uses its wings for swimming and is an egg- 

 parasite of dragon flies. Alaptus(Fig. 489) isoneof the smallest insects known. 

 Fam. 7. Chalcididae. Pronotum partly free and not reaching back 

 to the level of the wing-insertion. Antennae elbowed, of seven to thirteen 

 segments. No cells in wings, but one stout nervure which runs at first 

 parallel to, but finally falls into, the anterior border, after giving off a 

 slight twig. Trochanters divided. The Chalcids form a very large but 

 imperfectly known, clearly defined family of minute parasitic insects. Many 



of their larvae live in galls 

 preying on the more legitimate 

 inhabitants. Sometimes the 

 egg is laid in the caterpillars 

 and even in the pupae of Lepi- 

 doptera and often in the bodies 

 of Bees and Beetles. They help 

 to keep down swarms of injuri- 

 ous insects, and cases are on 

 record where they have de- 

 stroyed 95 per cent, of the eggs 

 of these enemies to cultivation. 

 The numerous species known as 

 fig-insects have frequently wing- 

 less males and winged females, 

 an unusual arrangement. Blas- 

 tophaga grossorum is the chief 

 agent in the process of caprifi- 

 cation, as the fertilization of the 

 fig is called. The insects which 

 live normally in the wild-fig are 

 allowed access to the cultivated 

 varieties. Unless the Blasto- 

 phaga be present some writers think that the fig will be but imperfectly 

 formed. Chalcids are often metallically coloured and of bizarre shapes. 

 The larva usually pupates within the body of its host. 



Fam. 8. Ichneumonidae. Long, straight, many segmented antennae ; 

 wings with many nervures and cells ; the cubital cell of the fore-wing is 

 separated from the second posterior by two cells. The trochanters are 

 divided into two segments. The Ichneumon-flies deposit their eggs in the 

 bodies of caterpillars and grubs by means of their long ovipositors ; and 

 myriads of insects are destroyed by ichneumon-larvae. The larvae are 

 said to eat, at any rate at first, the less essential organs of their host, 

 such as the fat body ; but in some cases at least they seem to live chiefly 

 on the fluids in their host's body. Ehyssa and Thalessa (Fig. 491) have 

 ovipositors of great length, and pierce solid wood with them in order to lay 

 their eggs in the burrows of the wood-boring larvae of the Siricidae (Fig. 488). 

 The number of species is very great ; over six thousand, about one-fifth 

 of which are British, have been described. Paniscus. 



Fam. 9. Braconidae. The cubital cell of the fore-wing separated from 



FIG. 491. Thalessa lunator. 

 (after Riley). 



Oviposition 



