

CHAPTER II 



ANATOMY AND PHYSIOLOGY 



i. SKELETON 



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

 ready known is about 400,000 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, 

 Tropidacris 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 casar 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, and a huge dragon fly (Meganeura) with 

 a spread of more than two feet. 



At the other extreme are beetles of the family Trichopterygidae, 

 some of which are only 0.25 mm. in length, as are also certain hymenop- 

 terous 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 

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

 unsegmented worm, the flexures of the body initiated by the muscular 

 system 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 



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