1S15.] On the Uses of the Dorsal J^essel. 375 



pillars the stigmata are equally formed of small cavities, pretty 

 deep, the edges of whicii are surrounded by a brown mark, and at 

 the bottom of which we discover a stripe of the same colour. 



A good deal might be said about the number and situation of 

 these stigmata, if we considered these organs in the different 

 orders of insects. But as these details would carry us too far, we 

 shall satisfy ourselves with observing, that the number of these 

 stigmata is so much the more considerable the more need the animal 

 has of air. Accordingly, in caterpillars we reckon 18 or 20 stig- 

 mata, and In a great number of the orthoptercs there are 12 or 16, 

 without reckoning the two tremaers. In certain species, which only 

 require a small quantity of air, the number of stigmata does not 

 exceed two; among the apteres they«wc/iCT/?\j exhibit this disposi- 

 tion. Besides these stigmata, we see in several orthopteres two 

 openings situated at the bottom of the abdomen ; but as their use 

 is not the same as that of the stigmata, we think it unnecessary to 

 describe them here. But we shall not pass over the large oval 

 opening which exists in the breast of the locust above the first pair 

 of paws. It communicates with a large trachea, which extends the 

 whole length of that paw. This opening serves too evidently to 

 introduce air into the superior part of the body, not to be consi- 

 dered as a true stigma; especially as it is formed like the otheir 

 stigmata of the extremities of the trachea;. As to the situation of 

 the stigmata, it is subject to numerous variations, always depend- 

 ing on the quantity of air which the insect requires. The more 

 air is required the more is the situation of the stigmata such as to 

 admit an easy introduction of it. The orthopteres, most of the 

 larviB, the lepidopteres, the hymcnopteres, and the diptercs, seem 

 the most favoured in this respect. In the caleopteres the trachefe 

 are generally placed not advantageously for the introduction of air. 

 Some of those that live in water are obliged when they wish to 

 respire to elevate a little their ehjtres, that the air may enter more 

 easily into their tracheai. 



The second kind of stigma, which we shall call trcmaer, is com- 

 posed of an oval opening, which opens and shuts by means of two 

 moveable horny pieces, set in motion by a peculiar apparatus of 

 muscles. The general form of that opening is that of an elongated 

 oval, the greatest diameter of which is from below upwards. In 

 expiration the moveal)Iu pieces separate from one another; they 

 shut on the contrary when the inspiration is terminated. These 

 moveable pieces open from witliin outward?, and do not separate 

 from each other more than half a millimetre, (0-0 1-90 inch.) The 

 movements of the tremaers agree sulTiciently with thoic of the 

 stigmata, and like them they connnunicatc with the trachea;. It 

 is even very easy in dissecting tluse parts to recognize the tracheae 

 which go to them, and the muscles which move the tremaers, two 

 in number, or one for each treinaer. These muscles arc destined 

 to separate or open the moveable pieces of the tremaers : accord- 

 ingly, they shut when these musclcB cease to act. These muscles 



