114 ESSENTIALS OF ZOOLOGY 



as nymphs. The imago is the perfect insect, and this stage 

 is usually a short one in duration. 



There may be several generations in a year, as in the case 

 of many insects which are harmful to plants of the garden and 

 the field ; typically the life-cycle is completed in a year, but 

 may occupy two years, or three years as in the case of the 

 beetles called in the larval state wireworms, four years in the 

 case of the cockchafer beetles, and the American locust, Cicada 

 septendecim, is said to live seventeen years. Only a few 

 insects in the imago state survive the winter of the northern 

 hemisphere. The plant lice, or aphides, reproduce partheno- 

 genetically all the summer, males appear in autumn, and the 

 fertilised winter egg remains in the unhatched state until 

 spring, when it ushers in again the generations of partheno- 

 genesis. 



Blatta. -The common cockroach, Blatta orientalis, has been 

 got in the wild condition only in the Crimea. Whether the 

 Crimea is its original home is questionable, but it is now spread 

 in kitchens and bakehouses in every part of the world. It 

 appeared in Western Europe in the sixteenth century, and its 

 spread has been produced by ship and land transport. 



Another genus, Periplaneta, with the well-known species 

 P. americana, is also common and cosmopolitan. It appears 

 to have originated in South America. It is brought to this 

 country by ships, and it is principally found in ships and at 

 seaports. The genus is easily distinguished from Blatta by 

 the greater size and by the large size of the wings, which are 

 longer than the abdomen in both sexes. P. australasiae is a 

 native of the East Indies, and is characterised by yellow bands 

 around the antennae and on the first segment of the thorax 

 and the elytra. This species has likewise become widely 

 spread. For further information with regard to these and 

 other species see Lucas (1920, ' Eay Soc. Monogr. of British 

 Orthoptera '). 



EXTERNAL CHARACTERS. The ectoderm secretes a chitinous 

 cuticle resolved into the thick segments and the thin membranes 

 between them. The cuticle is also secreted by the ectodermal 

 invaginations of the alimentary canal and respiratory tubes. 

 During the period of growth the cuticle is shed periodically, so 



