324 ARTHROPODA 



The largest group of arthropods, the insects, is distinguished by the 

 possession of wings, two pairs of which are typically present, arising as 

 projections from the dorsal wall of the thorax. 



The principal organs of special sense are the eyes, the tactile hairs, 

 and the auditory or balancing organs. The tactile hairs are usually dis- 

 tributed over the body, but are probably most sensitive on the antennae 

 and the palps, where they are organs of touch, hearing, taste, or smell. 

 The eyes are of two kinds, simple and compound. A simple eye or 

 ocellus is a minute structure formed by a modification of the integu- 

 ment and consisting of a convex retina and a lens. A compound eye is 

 a much larger and more complex structure and consists of a large number 

 of distinct elements called ommatidia. Each of these is a separate light- 

 perceiving body, and the sum of the images of all the ommatidia of a 

 compound eye forms the picture the animal sees. This is called mosaic 

 vision and characterizes crustaceans and insects alone among animals. 

 A pair of lithocysts, or so-called auditory organs, are present in certain 

 crustaceans; they are organs of equilibration. A few species of insects 

 have genuine auditory organs. 



The integument of arthropods is composed of a shell-like cuticula 

 which forms the entire outer surface, and a layer of glandular cells called 

 the hypodermis which lies beneath the cuticula and secretes it. The 

 cuticula is rendered tough and thick by the presence of chitin and 

 sometimes of calcium carbonate and forms a very efficient outer cov- 

 ering. During the period of growth an arthropod sheds its cuticula 

 periodically. 



Arthropods are often highly colored, some of them being among the 

 brightest of animals: many are protectively colored and many crustaceans 

 are transparent, being almost invisible in the water in which they live. 

 Sexual dimorphism is very common, the males being distinguished from 

 the females by size, color, or other external markings. 



Internal Structure. The digestive tract extends from the mouth to 

 the anus and is made up of foregut, midgut, 'and hindgut, which, however, 

 in all arthropods have undergone a large degree of specialization. Sali- 

 vary glands are generally present in the terrestrial arthropods but are 

 absent in the aquatic ones. Other digestive glands are wanting in insects, 

 but in crustaceans and arachnids a voluminous liver is often present. 

 The excretory organs or kidneys are present in the form of one or more 

 pairs of tubular glands. 



The circulatory system is not highly specialized. A blood fluid is 

 always present which circulates among the organs in the extensive body 

 cavity. In many small crustaceans no heart or other vessels are present, 

 but the circulation is maintained by the movements of the intestine or of 



