486 ARTHBOPODA. 



The sexes are readily distinguished by the genital armature. 

 The female isj^rovided witli the oYij)ositor already described, which 

 when used for this purpose only, projects from the hinder end of 

 the body (fig. 518), but when used as a sting (aculeus), can be 

 drawn back in the body when at rest. The sting, naturally lacking 

 in the male, is connected with a poison gland, the secretion of 

 which owes its efEect, not, as once believed, to formic acid, but to 

 a little known basic substance. 



The distinction between ovipositor (terebra) and aoaleus affords chprac- 

 ters of systematic importance; othersarefurnishedb}' the development, which 

 is always holometabolous. The pupae, in all important points, are similar 

 (pupae liberse), but two kinds of larvae are distinguished. Some have larvse 

 with well-developed legs ; many of them are green in color and distin- 

 guished from the larvae of Lepidoptera by the greater number of prolegs. 

 Others have footless larvae (fig. 59). The first occur where the larva must 

 shift for itself, the second where it is surrounded by an abundance of 

 food, either provided by the parents or by the host in which it is parasitic. 

 Sub Order I. TEREBRANTIA. Terebra present ; larvae with feet at 

 least on the thorax; eggs laid on leaves or in wood, usually without gall 

 formation; the larvae therefore must move in order to feed. The Tex- 

 THKEDINID.E, saw flies, feed on leaves and have caterpillar-like larva?. 

 Cimbex, Nematus. Siricid.e (Uroceridce), horn tails, the larva? bore in 

 wood and are whitish. 



Sub Order 11. ENTOPHAGA. Terebra present ; larvae legless, para- 

 sitic in galls or in animals. Some use the ovipositor to lay their eggs in 

 leaves, roots, or stems of plants. Galls are then produced, diseased struc- 

 tures by which the larvae are nourished. Others use the ovipositor to lay 

 their eggs on or in other insects and larvae. The young feed on the tissues 

 of the host and at last cause its death, often before the completion of the 

 rDetamorphosis. The gall-producing forms are the Ctnipid.e, some of which 

 afford cxiunples of heterogony (p. "145), in which the alternating generations 

 are distinguished by different structure, by sexual 

 and parthenogenetio reproduction, and by different 

 kinds of galls. So different are the two generations 

 that they have frequently been described as differ- 

 ent genera. The inquilines do not form galls, but lay 

 their eggs in the galls of other species. The insect 

 parasites are divided among several families, the more 

 prominent Ijeing the Iohn'rumomd.e. Bn.vcoxiD.E, and 

 C'HALciniD.E, tliosL! of the first being large, the others 

 Fio 51')— r7/n(c,-.s Hiiri- '^"'''" '"' uiiiuite. These forms are of immense value 

 pes.* (After Howaril.) to agiMcuUure, as they keep down, as no economic 

 entomologists or insecticides can, injurious forms. 



Sub Order III. AOULEATA. Females with stings ; larva- footless, 

 maggot-like. Tlio digger was|)S or Fossoues excavate tubes in the earth in 

 which they lay their eggs and then bring into the holes as nourishment 



