INSECTA 471 



a bed of chewed vegetable matter serves as the food for the royal pair 

 and the nymphs. 



The workers and soldiers differ from the sexual individuals, not 

 only in their sterility, but also in having more powerful mandibles. 

 In the soldiers the head can produce a protective secretion and the 

 mandibles are greatly specialized for defence (Fig. 326). Both these 

 castes consist of males and females, though secondary sexual cha- 

 racters are not very marked. 



If, as is stated, slight caste differences are already apparent in the 

 newly hatched young, caste-formation cannot be a matter of nutrition. 



Order PLECOPTERA (Stoneflies) 



This is a small order of mandibulate insects with a heterometabolous 

 metamorphosis. Though in possession of two pairs of well-developed 

 wings, they are weak fliers and do not move far from their aquatic 

 breeding grounds. Prominent, elongate antennae and cerci are cha- 

 racteristic features, as also are the three-jointed tarsi. According to 

 some authorities the wing venation represents a primitive type. Much 

 variation in venation is, however, found in the order. 



The nymphs are always aquatic, for the most part inhabiting swift- 

 flowing streams with stony beds. They possess the antennal and cereal 

 features of the adult and breathe by means of gill tufts in various 

 positions. In some cases gill vestiges are found on adults though these 

 are not aquatic. Like most aquatic insects, they have a wide distri- 

 bution, the most generalized families being found in southern, the 

 most specialized in northern, regions. Perla maxima is a common 

 species found in European streams. 



Order EMBIOPTERA 



Small insects with elongated and flattened bodies ; two pairs of similar 

 wings with reduced venation; females apterous; cerci two-jointed, 

 generally asymmetrical in male; metamorphosis absent in female, 

 slight in male. 



These insects are widely distributed in the warmer parts of the 

 world. Many are gregarious, living in tunnels formed of silk produced 

 by tarsal glands, e.g. Embia major from the Himalayas. 



Order PSOCOPTERA (Booklice) 



Small insects, either winged or wingless; with biting mouth parts; 

 thoracic segments distinct ; wings with reduced venation from which 

 cross veins are largely absent; metamorphosis slight. 



These insects are to be found on bark and leaves of trees and feed 

 on lichens and dry vegetable matter. The eggs are laid on the bark 



