ANTS, BEES AND WASPS 183 



and so do a great deal of harm to the plants on which they feed. 

 Some live inside fruits ; others in the stems of plants ; while some 

 roll themselves up in leaves or mine between the tissues. 



The Gall-flies, Cynipidcs, are minute insects, usually black or 

 very dark in colour. More than forty species attack the oak, 

 causing galls (among which is the familiar oak-apple) to form; 

 and the "bedeguar" or rose-gall is also caused by the agency 

 of one of these little insects. The female Gall-fly pierces the tissues 

 of the plant with her ovipositor, and in the wound inserts an egg. 

 As the resulting larva feeds within the plant the curious swelling 

 or gall forms round it, although the exact reason for this develop- 

 ment is not known. 



The Ichneumon Flies, of which nearly six thousand species have 

 been described, are parasitic in the larval stage on the larvae of 

 Lepidoptera, and in this way render good service in helping to 

 keep numerous insect pests in check. Most species live inside 

 the bodies of their victims and feed on the fatty tissues without 

 actually killing their hosts ; but the caterpillars become so weak- 

 ened that they die on changing to pupag, and the Ichneumon 

 larvae then issue from within and spin their tiny yellow cocoons 

 outside the pupa cases. It is a common sight to see the pupae 

 of the cabbage white butterfly entirely covered by the cocoons 

 of the parasite. The eggs are laid by the female on the outside 

 of the skin, in some cases, and the larvae on hatching bore their 

 way under the skin of the caterpillar. Other species of Ichneumon 

 pierce the skin of their victim with the ovipositor and lay their 

 eggs beneath it ; and in some species the larvae are parasitic 

 externally. Although the Ichneumon Flies chiefly attack the larvae 

 of Lepidoptera, other insects and spiders fall victims to certain 

 species. 



