118 AN INTRODUCTION TO ENTOMOLOGY 



thread (Fig. 135). The spiral thickening of the intima of a trachea is 

 termed the tcsmdium. In some insects there are several parallel 

 taenidia; so that when an attempt is made to uncoil the thread a 

 rit)bon-like band is produced, composed of several parallel threads. 

 This condition exists in the larger tracheae of the larva Corydalus. 



The epithelium of the trachea is a cellular layer, which is directly- 

 continuous with the hypodermis of the body -wall. 



The basement membrane is a delicate layer, which supports the 

 epitheliiun, as the basement membrane of the body-wall supports the 

 hypodermis. 



3. The Tracheoles 



The tracheoles are minute tubes that are connected with the tips 

 of tracheae or arise from their sides, but which differ from tracheae in 

 their appearance, structure, and mode of origin; they are not small 

 tracheee, but structiu-es that differ both histologically and in their 

 origin from tracheae. 



The tracheoles are exceedingly slender, measuring less than one 

 micron in diameter; ordinarily they do not taper as do tracheae; 

 they contain no taenidia; and they rarely branch, but often anasto- 

 mose which gives them a branched appearance (Fig. 136, t and 

 138 B, i). 



Each tracheole is of unicellular origin, and is, at first, intracellular 

 in position, being developed coiled within a single cell of the epitheliiun 

 of a trachea. In this stage of its development it has no connection 

 with the lumen of the trachea in the wall of which it is developing, 

 being separated from it by the intima of the trachea. A subsequent 

 molting of the intima of the trachea opens a connection between the 

 limien of the tracheole and the trachea. At the same time or a little 

 later the tracheole breaks forth from its mother cell, uncoils, and 

 extends far beyond the cell in which it was developed. 



The tracheoles are probably the essential organs of respiration, the 

 tracheae acting merely as conduits of air to the tracheoles. 



4. The Air-Sacs 



In many winged insects there are expansions of the tracheae, 

 which are termed air-sacs. These vary greatly in nimiber and size. 

 In the honeybee there are two large air-sacs which occupy a consider- 

 able part of the abdominal cavity; while in a May-beetle there are 

 hundreds of small air-sacs. The air-sacs differ from tracheae in 

 lacking taenidia. 



