I04 INJURIOUS AND USEFUL INSECTS 



insects is not restricted to the Hymenoptera. There is a large 

 family of Dipterous flies, the Tachinidas, which are reared 

 within caterpillars and other larvae. The Tachinid flies are 

 unable to pierce the skin of a larva ; they can only glue their 

 eggs to the victim, and leave the issuing grubs to force a 

 passage for themselves. 



It is the parasitic Hymenoptera which do most to keep 

 down the numbers of plant-eating insects. The name Ichneumon 

 calls for explanation. Greek naturalists knew of a weasel-like 

 quadruped which they called ichneumon, the mongoose of 

 India. Different species abound in parts of Africa and 

 tropical Asia, feeding on small quadrupeds, snakes, insects, 

 and eggs of several kinds. It was anciently believed that the 

 ichneumon of Egypt was serviceable in devouring the eggs of 

 the crocodile, and it was reputed bold enough to enter the 

 crocodile's throat, and feast upon its entrails. Coming to 

 know of wasp-like insects, which preyed upon spiders and 

 their eggs, the Greeks likened them to ichneumons, and called 

 them by the same name. 



The parasitic Hymenoptera (or Ichneumons and their allies) 

 may be recognised by the following marks : — The waist is 

 usually narrowed, sometimes to such a degree that the abdomen 

 looks like a mere knob on the end of a long tail. The leg has 

 a double trochanter, the joint so named being divided into 

 nearly equal parts by an articulation. This peculiarity occurs 

 in some other Hymenoptera — e.g. in Sirex. There is no sting, 

 but the same parts which compose the sting of the bee are 

 here converted into a borer or egg-laying tube, like that of 

 Sirex. In most ichneumons, but not quite in all, the feelers 

 execute rapid quivering movements, which seem to be con- 

 nected with the search for hidden victims. The larvse are 

 footless grubs, the pupse are commonly protected by a cocoon, 

 which may be spun either inside or outside the dead body of 

 the host. Dr Sharp * thinks that there may be 200,000 species 

 of parasitic Hymenoptera, and far higher estimates of the 

 number have been made. Some of the families include gall- 

 makers and plant-eaters, as well as insect-parasites. 



The narrowed waist is not universal in Hymenoptera. Saw- 

 flies and wood-wasps, as we have seen, have a sessile abdomen, 

 as broad at its base, or nearly so, as in any other part. But it 



* "Cambridge Natural History,'' vol. v. p. 520. 



