PHYLUM ARTHROPODA. INSECTS 



249 



Respiratory system 



The respiratory system of insects (Fig. 

 137) is in general like that of the grass- 

 hopper, but modifications occur in many 

 species, especially in the larvae of those that 

 live in water. Aquatic naiads, in many cases, 

 do not have spiracles but get oxygen by 

 means of threadlike or leaflike outgrowths 

 at the sides or posterior end of the body, 

 termed tracheal gills. 



Special adaptations 



One has only to study the structure, 

 physiology, and behavior of an insect to dis- 

 cover adaptative modifications. A few in- 

 teresting examples are: (1) the walking stick 

 (Fig. 149) not only resembles a dead twig 

 but has the habit of feigning death; (2) the 

 male cricket possesses a highly differentiated 

 sound-producing apparatus consisting of a 

 file (Fig. 149) on the base of one wing and 

 a scraper on the other; when the wings are 

 held up over the body, the file is rubbed over 

 the scraper, producing the pleasant call of 

 the cricket. The rate at which these calls are 

 made is proportional to the temperature. 

 The significant part of the sounds produced 

 by some classes of insects is a frequency so 

 high as to be inaudible to the human ear; 

 (3) dragonfly naiads breathe by means of 

 rectal gills which line the enlarged posterior 

 end of the digestive tract and remove oxygen 

 from the water that is drawn in and expelled 

 from this cavity; the labium of the naiad 

 (Fig. 149) is much elongated and can be 

 extended rapidly from its folded resting po- 

 sition beneath the head so as to impale its 

 prey on the hooks at the end; (4) mosquito 

 larvae obtain air through a tube that is 

 thrust through the surface of the water; 

 (5) water striders (Fig. 149) have long, 

 slender legs which do not break through 

 the surface film as they skim about over the 

 water; (6) fireflies are provided with an or- 

 gan capable of emitting light; the females 



and larvae, known as glowworms, are also 

 luminescent; (7) click beetles leap (click) 

 by means of the action of a prosternal proc- 

 ess in a metasternal groove; (8) dung 

 beetles, including the sacred scarab of the 

 Egyptians (Fig. 149), roll up balls of dung 

 in which an egg is laid and on which the 

 larva feeds; (9) the larvae of most caddis- 

 flies (Fig. 149) build portable protective 

 cases of sand grains or vegetable matter 

 fastened together with silk; (10) certain 

 hornets build nests of wood pulp (Fig. 149) 

 —they were the first papermakers; and (11) 

 gall wasps stimulate plants to develop ab- 

 normal growths called galls, presumably 

 caused by growth-stimulating substances 

 secreted by the insect. 



Hormones 



It is now well established that hormones 

 play an important part in the regulation of 

 the activities of insects, for example, hor- 

 mones control both metamorphosis and 

 molting. Experiments have shown that the 

 retention of juvenile characters and develop- 

 ment of adult structures are controlled by 

 hormones secreted by the brain, prothoracic 

 gland, and corpora allata. The corpus al- 

 latum is a gland which lies behind the brain. 

 If this gland is removed from the nymph 

 of the bug Rhodnius, at a certain time, 

 molting is prevented. 



COLORATION 



Everyone knows that many insects are 

 brilliantly colored, especially the butterflies, 

 moths, and beetles. Coloration of some in- 

 sects differs with the season, and one brood 

 may have one color pattern and a later 

 brood a very different one. Such insects are 

 said to be seasonally dimorphic (two types), 

 trimorphic (three types), or polymorphic 

 (more than three types). In certain species 

 the males and females are differently colored; 



