274 



METAZOAN PHYLA 



for swimming by being broadened and paddle-like and having their 

 area increased by hairs along the margins; or in the case of other aquatic 

 insects, the middle pair of legs is elongated and used like a pair of oars. 

 Some burrowing insects have the forelegs modified for digging; in the 

 mole cricket they have a curious resemblance to the forefeet of the mole. 

 In the mantids, and also to a lesser degree in the walking sticks, the fore- 

 legs are increased in size and fitted for grasping prey. Many insects 



C/aw 



Pulvi/los 



Fig. 173. — Modifications of legs in insects. A, praying mantis, Stagmomantis Carolina 

 (Linnaeus), to show forelegs modified for holding prey. Natural size. B, water boatman, 

 Corixa sp., with hind legs used as oars. X 2. C, part of under surface of a water beetle, 

 Dyliscus sp., to show hind leg modified as a paddle. X ^o. -D, part of under surface of a 

 ground beetle, Calosoma scrutator Fabricius, to show hind leg fitted for running. X l^lt- 

 E, fore part of the body of a mole cricket, Gryllotalpa hexadactyla Perty, to show digging 

 forelegs. About natural size. F, foot of a house fly to show claws and pulvilli, or pads 

 of hairs, together with longer hairs, all for clinging. Highly magnified. {A to E from 

 specimens; F from Kellogg, " Am,erican Insects," hy the courtesy of Heiiry Holt & Company.) 



cling to walls and ceilings by means of pads of hairs at the tips of the legs. 

 When these are pressed against a surface the air is forced out and the pad 

 holds by suction. 



The great majority of insects have wings, though there are wingless 

 types in a great many of the orders. The wings represent outfoldings 

 of the wall of the body in which the two sides of the fold have come into 

 contact and have grown together. Along certain lines, however, they 

 have failed to unite, and in these places chitinous tubes are developed. 

 These are called veins, or nervures; they serve to strengthen the wing and 

 divide its surface into areas called cells. While wings are growing and, in 



