CHAPTER II 

 ANATOMY AND PHYSIOLOGY 



i. SKELETON 



Number and Size of Insects. The number of insect species al- 

 ready known is about 300,000 and it is safe to estimate the total number 

 of existing species as at least one million. 



Among the largest living species are the Venezuelan beetle, Dynastes 

 hercules, which is 155 mm. long, and the Venezuelan grasshopper, A cri- 

 dium latreillei, which has a length of 166 mm. and an alar expanse of 240 

 mm. Among Lepidoptera, Attacus atlas of Indo-China spreads 240 mm. ; 

 Attacus ccBsar of the Philippines, 255 mm.; and the Brazilian noctuid 

 Erebus agrippina, 280 mm. Some of the exotic wood-boring larvae attain 

 a length of 150 mm. 



The giants among insects have been found in the Carboniferous, from 

 which Brongniart described a phasmid (Titanophasma) as being one-fourth 

 of a meter long. 



At the other extreme are beetles of the family Trichopterygidae, some 

 of which are only 0.25 mm. in length, as are also certain hymenopterous 

 egg-parasites of the families Chalcididae and Proctotrypidae. 



Thus, as regards size, insects occupy an intermediate place among 

 animals ; though some insects are smaller than the largest protozoans and 

 others are larger than the smallest vertebrates. 



Segmentation. One of the fundamental characteristics of arthro- 

 pods is their linear segmentation. The subject of the origin of this seg- 

 mentation is far from simple, as it involves some of the most difficult 

 questions of heredity and variation. As arthropod segmentation is 

 usually regarded as an inheritance from annelid-like ancestors, the sub- 

 ject resolves itself into the question of the origin of the segmented from 

 the unsegmented " worms." Cope, Packard and others give the me- 

 chanical explanation which is here summarized. In a thin-skinned, un- 

 segmented worm, the flexures of the body initiated by the muscular sys- 

 tem would throw the integument into folds, much as in the leech, and 

 with the thickening of the integument, segmentation would appear from 

 the fact that the deposit of chitin would be least at the places of greatest 

 flexure, i. e., the valleys of the folds, and greatest at the places of least 



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