676 



CLASS IV. INSECTA. 



feeble mouth parts, also two or three ocelli and moderately large 

 compound eyes (Fig. 426). The thoracic segments are distinct, 

 and, correlated with the gap between the bases of the legs, the 

 sterna are well developed. The abdomen has ten distinct seg- 

 ments and often ends in a pair of long, jointed cerci. The legs 

 are slender and have three tarsal segments. The tegmina lie 

 flat on the back, one overlapping the other, and each bent down 

 at the side so as to protect to some extent the sides of the body. 

 In opposition to the usual rule the wings of the males are often 

 reduced and smaller than those of the female. 



The oesophagus is large and there is no distinct crop or 

 proventriculus. The true stomach is small 

 with ten lobes. The malpighian tubes are 

 numerous. The nervous system comprises a 

 small brain, three thoracic and six abdominal 

 ganglia. Both male and female gonads 

 form a ring like those of many Arachnida, 

 and the latter bears a receptaculum on its 

 course. 



The Plecoptera lay very numerous eggs, 

 some thousands, each with a remarkable 

 micropyle. The eggs are said to be carried 

 about in a hollow on the ninth abdominal 

 segment of the female before being deposited 

 in the water. The larvae (Fig. 426) seem to 

 prefer running streams and even torrents ; 



and the adults > since the y are not strong on 

 the wing, usually frequent the vicinity of 

 such waters. The larvae are mostly found under stones ; they 

 have no stigmata and either absorb oxygen through the skin 

 or through tuft-like gills plentifully supplied by tracheae. Such 

 gills may persist until the imago instar is reached, and may 

 coexist with functional stigmata, of which there are usually two 

 pairs on the thorax and six on the abdomen. In the last 

 aquatic or nymph stage the insect resembles the aerial adult, 

 except in having no wings ; but like the larva it is active and 

 feeds on soft-bodied insects such as May-flies. 



On the whole the Order is perhaps most closely connected 

 with the Orthoptera and especially with the least specialized 

 members of the family Phasmidae. It comprises but one family : 



FIG ' 



