THE PHYLA ARTHROPODA AND ONYCHOPHORA 



specific effect of low temperature is to activate neurosecretory cells in the 

 brain of the pupa, causing them to secrete the hormone which stimulates pro- 

 duction of the prothoracic gland hormone; this, in turn, initiates development 

 of the adult insect within the pupal skin. 



Students of insect physiology are actively investigating such fundamental 

 questions as the evolution of these control mechanisms among insects in gen- 

 eral, the precise biochemical foundations of endocrine regulation of meta- 

 morphosis, and related problems. 



Representative Orders of Insects. Opinions differ about the number 

 of orders among which the members of the class Insecta should properly be 

 distributed. The number generally recognized varies approximately between 

 twenty and thirty. In this section we shall describe and discuss briefly only 

 a few of these orders, selected as illustrating the features of both simple and 

 more complex insects. Additional details concerning these and other orders 

 can be found in textbooks of entomology. 



It will be noted that the features most commonly used in distinguishing 

 between orders are (1) kind of life cycle; (2) nature of the wings, where these 

 are present; and (3) differentiation of the mouth parts for different methods of 

 feeding. A comparison of the various orders shows that the more generalized 

 types of insects possess mandibulate mouth parts. In those which have 

 developed other methods of feeding, involving perfection of suctorial mouth 

 parts, the modified feeding apparatus is clearly derived from the more primi- 

 tive mandibulate plan. In the simplest insects, such as Protura and 

 Thysanura, wings are absent in the immature as well as in the adult stages; 

 these and other wingless forms are sometimes grouped together as the Aptery- 

 gota. Other insects are wingless as adults but show, by the appearance of 

 abortive wing rudiments in the larval stages, that they have evolved from 

 winged ancestors. The presence of two pairs of functional wings is char- 

 acteristic of the Pterygota, comprising the majority of modern insects. In 

 Coleoptera, and to a lesser extent in Orthoptera and Hemiptera, the anterior 

 wings are specialized as protective covers for the hind wings. This, like the 

 reduction of the posterior pair of wings in the Diptera, is regarded as a modi- 

 fication of the more typical four-winged state. As previously stated (p. 454), 

 the simple, ametabolous life cycles of primitive insects indicate that the more 

 complex cycles of higher orders have developed with the evolution of the class. 



Order Protura (first tail) — minute, wingless forms living in humus and 

 decaying leaves (Fig. 15.25). These must be considered the most primitive of 

 living insects; although they possess a well-defined thorax and three pairs of 

 legs, they are without antennae, the anterior pair of legs being held in front 

 of the head as tactile organs. Unlike the more typical insects, proturans 

 possess vestigial appendages on the anterior abdominal somites. In the course 

 of growth to maturity, new somites are added at the posterior end of the body; 

 this kind of growth is not typical of insects generally but is found in more 

 primitive arthropods and in annelids. The mouth parts of proturans are 

 mandibulate, and the life cycle is ametabolous. 



457 



