S76 On the Uses of the Dorsal Vessel. [May, 



are composed of fibres sufficiently distinct, which are attached in 

 the second cavity of the thorax in the erismes. Hitherto we have 

 observed the tremaers only in a certain number of orthopteres, 

 where they present very various forms. In general, however, they 

 exhibit the arrangement of which we have spoken ; and ii is only 

 in the mantes that we see them situated on the lateral and externa/ 

 side of the thorax, between the superior and inferior portion of 

 that part. The tremaers have then a triangular form. Instead of 

 presenting two moveable pieces, they have only a single piece 

 moved by a particular muscle. It is always by means of this 

 muscle that the moveable part is raised up ; and as soon as the 

 muscle ceases to act, the moveable part falls down and shuts. The 

 membrane situated below the tremaer is so thin, that we can see in 

 the expirations and inspirations the elevation and depression of the 

 trachea, in proportion as the air enters or goes out. 



Tl\ough the position of the tremaers experiences some variations, 

 being sometimes situated in the neck, sometimes in the breast, 

 this is not the case with the number of these parts. We never 

 see more than two, the size of which is always proportional to the 

 quantity of air which the insects respire. Besides that manner of 

 receiving air the insects swallow it likewise by the mouth. What 

 is obtained in that way indeed can only make its way to the organs 

 of nutrition ; and as we have elsewhere explained its influence on 

 digestion,* I shall not resume the subject at present. The last 

 mode of respiration which certain insects present, is to have their 

 stigmata placed at the anus. Those exhibiting that organization 

 are chiefly the insects that decompose water, as the larvae of libel- 

 lulas and dytics. These stigmata, or the openings which allow the 

 •water to escape, are surrounded by small triangular and moveable 

 pieces, the principal use of which is probalily to keep away those 

 bodies that might hinder the introduction of water into these stig- 

 mata, and at the same time to shut the opening exactly when the 

 insects suspend this introduction. Accordingly, when these insects 

 choose to introduce water into their respiratory organs, they sepa- 

 rate the moveable pieces of which we have spoken, and they shut 

 them ill the contrary case. But when these pieces are separated 

 we easily distinguish the round opening by which water is intro- 

 duced into the respiratory apparatus. This opening presents a 

 diameter of about a ir)illimetre, 0-03957 i"th,) and it is easy by 

 means of it to let out the water which may exist in the interior of 

 the body. 



Considered with respect to tlieir respiratory organs, insects form 

 three distinct classes; namely, those that breathe air immediately; 

 those that live in water and are obliged to come ta the surface of 

 that liquid in order to receive the impression of the air, though 

 they likewise lay hold of that contained in the water; and those 



• See my Mpsioir on the IntesCmal Tube of Insccli, ingorteU in tlic Annales dii 

 Maicuin d'Hiiloire KatiuelJc, torn. lU. 



