FEOM EGG TO INSECT SNODGEASS 



407 



The tracheae are in<rrowths of the ectoderm and appear first as 

 pits along the sides of the body. The pits deepen into tubes, the 

 tubes fork out into branches, the branches subdivide until the 

 tracheae from each opening, or spiracle, become a finely branched 

 system like the roots of a plant. In most insects, some of the first 

 branches unite with corresponding branches 

 from successive and opposite spiracles, thus pro- 

 ducing lengthwise and crosswise connecting 

 trunks. 



It is not certain whether the primitive spir- 

 acles are to be regarded as segmental or inter- 

 segmental in origin; they are later subject to 

 considerable shifting of position, though in 

 general they remain on the sides of the seg- 

 ments. There are usually two pairs on the 

 thorax and eight on the abdomen in adult 

 insects (fig. 29 Sp), but there is evidence to 

 suggest that there was originally a pair of 

 spiracles to each of the segments. The tracheae, 

 being of ectodermal origin, are all lined 

 Avith a continuation of the cuticula of the 

 body wall. 



ECTODERMAL GLANDS AND CENOCYTES 



A great variety of glands are found in 

 insects that open on the surface of the skin or 

 parts derived from the ectoderm. Chief among 

 these are the salivary glands opening near the 

 mouth and the glands of the female that form 

 a substance used either to glue the eggs to a 

 support or to make a covering for them. 

 Other glands, however, form disagreeable- 

 smelling substances, the odor of which is sup- 

 posed to ward off enemies, and some glands 

 secrete substances that are used as food by 

 other members of the species or by members of 

 other associated species. Honeycomb is a waxy 

 product of glands in the Avail of the abdomen of worker bees, 

 and the same bees feed the brood with a " royal jelly " secreted by 

 glands opening into the mouth. All these glands are simply ecto* 

 dermal cells that have acquired special functions. Some glands are 

 one-celled (fig. 23 A, GlCl) and remain flush with the surface, 

 perhaps penetrated by a tubular ingrowth of the surface cuticula, 



Fig. 22. — Respiratory 

 system of a cater- 

 pillar, showing the 

 large tracheal trunks 

 (a) along sides of 

 body, the crosswise 

 connectives (6), the 

 principal smaller 

 branches in head (H) 

 and in the lower half 

 of body, and the se- 

 ries of breathing ap- 

 ertures, or spiracles 

 (Sp), on each side. 



