THE ORGANISM AND ITS ENVIRONMENT 357 



wing membrane. The characteristic call of this insect is emitted by 

 the rubbing together of these two areas, the air under the covers 

 serving as a sound box. In the cricket each wing cover is provided 

 with a device resembling a file and another resembling a scraper. 

 When the chirp is emitted the wing covers are elevated at an angle 

 and rubbed together, the scrapers and files being in such position 

 as to cause the wing covers to vibrate. The whining or buzzing 

 noises produced by flies, mosquitoes, bees and wasps, and other 

 flying insects, result from the rapid beating of the wings against the 

 air. In bees and wasps the sound may be produced while the animal 

 is moving about on foot, but the flies and mosquitoes emit a buzz 

 or whine only when in flight. 



The Class Insecta As an Example of Divergence within 

 a Related Group. Insects, because of their tremendous number 

 of varieties, illustrate more than any other group the principle of 

 divergence in adaptation. So numerous are the forms and so com- 

 plex are the life histories, that the study of insects. Entomology, con- 

 stitutes a highly technical special branch of Biology. The economic 

 importance of insects lends special interest to their study. 



The class Insecta is distinguished from other Arthropoda by the 

 following characters: The body is divided into three portions, head, 

 thorax, and abdomen (Fig. 60). There are three pairs of walking 

 legs always present and always attached to the three segments that 

 compose the thorax. The breathing apparatus consists of a set of fine 

 tubules, the tracheae, which ramify throughout the tissues and open 

 to the air through apertures, termed spiracles, along the lateral walls 

 of the thorax and abdomen (p. 168). Wings are not always present; 

 when present there are either one or two pairs, always attached to 

 the most posterior, or to the middle and posterior segments of the 

 thorax. Insects are often confused with spiders, the class Arachnida, 

 which have four pairs of walking legs, never have wings, and 

 usually breathe by means of book lungs (p. 167). 



