ARTHROPODA 293 



The complex instincts of many insects as yet defy our anal- 

 ysis. We do not know to what extent these instinctive actions 

 are produced by external and by internal conditions, but we do 

 know that external stimuli furnish the occasion; for example 

 decaying meat initiates the egg laying reflex in gravid flies. The 

 most remarkable instincts are connected with the place and 

 time of egg laying, the building of homes, the capturing of prey, 

 and the complex social life of the higher forms. The student 

 can well afford to seek illustrations of these. 



In some of the Crustacea there is an interesting power of 

 breaking off an injured leg at a definite point. This is not because 

 of particular weakness at this point, but is the result of definite 

 muscular contractions. It is a reflex response to certain stimuli. 

 The regeneration of the leg occurs from the stump. Many 

 crustaceans have the power of regenerating any of the append- 

 ages which may be lost. This is true also of the eyes, which are 

 on stalks. Under certain circumstances antennae may develop 

 in place of the eyes. 



334. Library Exercises. — Reports on the social life of bees and ants; on the 

 animals captured and utilized by ants; on power of flight in ants; on queens among 

 ants and bees; on myrmecophilous (ant-loving) insects; on intelligence among 

 insects and spiders. 



335. Classification. 



Class I. Crustacea {with shell; Crayfish, Crabs, Barnacles, etc.). — Arthropoda, 

 chiefly inhabiting the water and breathing by means of gills or through the body 

 wall. The head typically consists of five segments more or less fused and bearing 

 two pairs of antennae or feelers, one pair of mandibles, and two pairs of maxillae. 

 The thorax or second region of the body may be separate from or fused with the 

 head {cephaloth'orax). It possesses a variable number of segments, which usually 

 bear the locomotor appendages. The remainder of the body (abdominal segments) 

 is normally of distinct segments in which the appendages are much reduced. The 

 chitinous skeleton is ordinarily well developed. 



Subclass J. Entomostraca. — Crustacea, small and simple in organization, with 

 a variable number of segments of which the appendages are simple and less diverse 

 than in the next subclass. Many of them are parasitic and degenerate. A meta- 

 morphosis occurs. The group embraces many small free forms, found both in 

 fresh and salt water, some fish parasites, and the sedentary barnacles. Here 

 belong Cyclops and Daphnia, which occur abundantly in fresh water pools and feed 

 on the algae common there. They constitute an important portion of the food of 

 the fresh-water fishes. They multiply very rapidly and keep closely up to the 

 limit of the food supply. The eggs of many of them can resist drying to a re- 

 markable degree. This is of manifest importance when we remember that they 

 frequent pools in which drouth is not uncommon. 



