466 THE INVERTEBRATA 



Class PTERYGOTA 



Subclass EXOPTERYGOTA 



Order ORTHOPTERA 



Insects with generalized biting mouth parts; ligula 4-lobed, con- 

 sisting of inner paired glossae and outer paraglossae ; fore wings rather 

 narrow and somewhat hardened {tegmina) ; hind wings membranous ; 

 abdomen usually with jointed cerci of short or moderate length; 

 ovipositor generally present. 



This order comprises terrestrial insects of large size which have 

 great powers of running and jumping. There are many flightless 

 species in all the families (cf. the female of Blatta orientalis). 



The main structural features are exemplified by Periplaneta, the 

 cockroach. Its generalized character is shown by the character of 

 the mouth parts, the nervous system (six abdominal ganglia), the 

 circulatory system (heart with thirteen chambers, three in the thorax 

 and ten in the abdomen), and the obvious ten segments of the 

 abdomen. 



The order is divided into two sub -orders, the Cursoria in which the 

 legs are of approximately equal size and the Saltatoria in which the 

 last pair of legs are modified for jumping (Fig. 324). The former 

 consists of the Blattidae (cockroaches) which are swift-running, 

 omnivorous forms, usually tropical in their distribution, the Mantidae 

 (praying insects), which are carnivorous, with modified raptorial fore 

 legs, and the Phasmidae (stick and leaf insects), some of which are 

 immensely elongated and attenuated to resemble sticks or twigs, while 

 others have laminar expansions of the skin that give the animal a re- 

 semblance to leaves, which is closer in the female than in the male. 

 The female phasmid at any rate is almost motionless, and the habit of 

 feigning death is commonly developed in the family. All these cha- 

 racters help to protect the female from observation in the plants 

 which it frequents and of which it eats voraciously. 



In the Saltatoria there are the Acridiidae (locusts and short-horned 

 grasshoppers), the Locustidae (long-horned grasshoppers), and the 

 Gryllidae (crickets). The latter include a form remarkably adapted 

 for a burrowing life, namely Gryllotalpa. Nearly all these insects are 

 vegetarian, and in the Acridiidae, while the species commonly live 

 a solitary existence and are harmless, under certain conditions a form 

 with a gregarious and migratory instinct develops in countless num- 

 bers which invade cultivated districts causing incalculable harm. 



Thus in the case of Locusta migratoria, when environmental con- 

 ditions favour an increase in numbers, there is an inevitable trend 

 towards the production of swarming migrants, i.e. the gregarious 



