414 THE INVERTEBRATA 



Up in the form of glycogen. In some way, not properly understood, 

 the termite takes advantage of the carbohydrate, thus made digestible, 

 and if, as is possible by raising the temperature to 40° C, the termites 

 are ''deprived" of their protozoa, wood is no longer digested and the 

 animals starve. Cultures of termites have been kept for more than a 

 year with their associated trichonymphids on a diet of pure cellulose 

 (filter paper). It is difficult to conceive how they obtain nitrogen for 

 building up body proteins. 



Termites may forage by night for plant food and the genus Termes 

 also cultivates in its n^st fungus gardens . The fungus which grows on 

 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 speciaHzed for defence (Fig. 307). Both 

 these castes consist of males and females, though secondary sexual 

 characters 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 

 characteristic 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 distribution, 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. 



