36 FORM ANt) COLOUR; 



whilst the small forms have an enormously rapid power of multiplication. 

 In any one species the size is usually constant, but is not a character 

 that can be used to discriminate species. Some species are very variable 

 in size, and males are frequently smaller than females. 



Form. 



The fact that an insect's form is correlated with its habitat and by 

 the necessities of procuring food is abundantly manifest. Insects in 

 general are somewhat cylindrical, a form that allows of twisting and 

 flexible movements. This persists in flying insects 

 (tig. 43), as offering less resistance to the air and is 

 also well seen in most larvae. Boring insects are 

 extra-cylindrical, as it were, fitting tightly in the 

 tubular burrows they make or, as in the beetle grubs, 

 having a special cushion-like projection that fits the 

 burrow tightly and facilitates locomotion (fig. 88). 



Many insects are flattened; bark beetles and 

 similar insects that find food between the bark and 

 the wood are often flat, to the extreme of being 

 leaf-like. Ground beetles, cockroaches (fig. 45) and 

 other insects that live on the ground, hide under stones, and run fast, are 

 usually flattened. So too are some caterpillars (fig. 47) that cling tightly 

 to the leaves of plants, and leaf-miners which find their food between 



the upper and lower epidermis 



of a leaf. The tortoise beetles 



FIG. 45. 

 Common Cockroach. 



FIG. 46. 



The Buy ; an example of a flat insect* 

 (Magnified,) 



are nat and can 



to a smooth leaf after the manner 



mantises are often formed like a 

 grass stem, and lurk in the grass in 

 the hope of unwary insects mistak- 

 ing them for a grass stem and so 

 getting within their reach. Others 

 resemble dry sticks and live on dry 

 bushes. The green grasshoppers 

 (fig. 29) for a similar reason are 

 formed so as to suggest a green 

 leaf and deceive butterflies. Mag- 

 gots that live in decaying matter 

 are smooth and worm-like, with 



