2 GUIDE TO CALIFORNIA INSECTS 



which cannot well be separated from insect parasites. 

 CLx\SSES OP ARTHROPODA. 



Insecta: winged, or one pair of antennae and three pairs of legs, or if 

 legless, with tracheae. 



Arachnida: without antennae. 



Crustacea: with two pairs of antennae. 



Myriapoda. Onychophora: legs not definitely jointed. 



The plant feeding forms of all these classes are always treated by economic 

 entomologists as tho they were insects, the most important group being the 

 red spiders among the arachnida; of lesser importance are the sow bugs 

 among Crustacea, and the millipeds of the Myriopoda. 



The Structure of Insects is identical in its general features with that of 

 the other classes of Articulata. The exterior of the body is hardened to give 

 attachment to the muscles and serves as the skeleton as well as skin. The 

 hard parts are the secretion of a delicate layer of cells called the hypoder- 

 mis, which develops originally in the egg as a flat disc from which most of 

 the organs of the body, internal as well as external, arise. 



This disc is called the ventral plate because it is converted directly into 

 the breast of the insect, cells around tho edge multiplying and giviDg rise to 

 the remaining parts of the body. 



The first differentiation of the ventral plate is the splitting off from the 

 inner side of two ribbons of cells which become the nervous system, lying 

 as a double chord along the middle line beneath. These produce long slen- 

 der filiments, the nerves, which grow out to every muscle and sense organ 

 as they are developed. The cells from which these proceed gather them- 

 selves into a regular series of knots which early unite in pairs forming 

 ganglia along the double ventral chord. 



The ventral plate becomes at the same time marked off into a series of 

 areas, one corresponding with each ganglion. As the ventral plate grows and 

 finally closes over the back, these areas form rings or segments of the body. 

 The cells between these areas not only become somewhat different in ap- 

 pearance but finally secrete a more delicate and flexible cuticle than those 

 forming the general surface of the segment. These thin portions of the 

 skin permit the body to bend between the segments. 



Long before the segments are completed and even before the two halves 

 of the ganglia have come together there begins an outgrowth opposite each 

 forming ganglionic mass, showing first as slight elevations of the surface but 

 becoming long and tubular. Soon differentiations of the cells of these pro- 

 cesses similar to those which produce the segments, result in the production 

 of jointed appendages as indicated by the name Arthropoda. 



In the appendages which become the legs the first articulation to become 

 evident is the knee joint. The adjacent segments are named after human 

 anatomy, femur and tibia. 



