6 GUIDE TO CALIFORNIA INSECTS 



in the development of the embryo, a few cases being ivnown in which they 

 are not developed so as to be functional at hatching. The invaginations are 

 formed on either side of the segment. Some of the segments always fail to 

 produce tracheae and in such cases the tracheae of adjacent segments sup- 

 ply the lack. The parts of the tracheal system are as follows: — 



1. Spiracles or stomata, at the point of invagination there usually develops 

 an elaborate structure bordered by one or more chitinous rings, within which 

 dense combs of spines stand guard over the slit-like opening. Within the 

 body the trachea may expand into a subspherical chamber and may be pro- 

 vided with chitinous bars with muscle connections enabling the insect to 

 completely close the opening. 



2. Spiracular trunks, being the portion of the tracheal system developed 

 before the proximity of the digestive tract resulted in the stopping of the 

 growth at the tips of the invaginations and the beginning of growth at two 

 points like adventitious buds for the production of the lateral trunks. 



3. Lateral trunks, resulting from the joining of the whole series of out- 

 growths on a side into one long tube extending almost from end to end of 

 body. The points of union are at first closed just as are those of the three 

 regions of the digestive tract and there always remains at these points a 

 ring of entirely different structure from the surface of the tracheal wall. 



4. Dorsal and ventral girdles, developing at various points along the lat- 

 eral trunks, and varying greatly in number and position in different insects. 

 These arise as pairs of large tracheal trunks growing around the digestive 

 tract, meeting and joining in the same way that the elements of the lateral 

 trunks grew together. 



5. Branches, which arise at various places along any of the trunks, but 

 chiefly at or near the point where the first forking occurred which subdivide 

 into a great number of very fine thread-like tubes which proceed to invade 

 every tissue and organ and show no tendency to anastomose. 



There are in addition to the digestive tract and tracheae numerous glands 

 opening thru the skin in various parts of the body in different insects, which 

 arise as invaginations of the skin, usually rather late in the development of 

 the insect. The ducts of the sexual organs also belong to this category. 

 There is first produced a pair of invaginations which are finally pinched off 

 entirely from the skin and finally attach themselves to the end of a single 

 median ingrowth. The first egg therefore passing out from the ovary must 

 first break thru its own investment of connective tissue and into one of the 

 lateral ducts and then thru the walls separating this from the common 

 duct. A special organ to tear the way thru these tissues is developed below 

 the lowest egg in an ovariole. 



The sense organs of insects consist of: — 



1. Eyes located on the head, usually a pair of compound eyes and three 

 simple eyes. 



