THE INTERNAL ORGANS OF RESPIRATION. 173 



Lamellicornia, in which, according to these entomotomists, the spiral 

 filament is deficient, whereas others, particularly Suckow and Sprengel, 

 assert that they exist, of which we shall speak in detail below. 



128. 



With respect to the differences of form in the tracheae, according to 

 Marcel de Serres they may be divided into three main groups, which 

 that writer thus distinguishes : 



1. ARTERIAL AIR-VESSELS. They originate directly from the 

 spiracle, and ramify with the most delicate branches from this simple 

 stem to all the internal organs. 



2. TUBULAR or PULMONARY AIR-VESSELS. They do not receive 

 the air directly, but stand in connexion with the spiracle by means of 

 the former. They are larger than the arterial air-vessels, their course 

 is more regular and straight, their diameter broader, and their branches, 

 on the contrary, smaller. 



3. VESICULAR AIR-VESSELS. They are of two kinds, either large 

 bladders, in which the air collects, and whence the branches spring, or 

 small bladders in the branches themselves, and frequently the terminal 

 distended ends of the branches ; both forms are never found together. 



Upon inspecting first the arterial air-vessels, as those most generally 

 found, but little that is extraordinary is to be remarked in them ; each 

 main stem originates from the internal margin of each spiracle with a 

 broader base, which narrows somewhat after a short course. Here also 

 is the point of division of the main stem ; next a branch spreads for- 

 wards and backwards, which passes to the anterior and posterior spiracles 

 to unite with each main stem originating from them. By means of these 

 arches all the stems of the tracheae stand in close connexion together. 

 Between these two communicating tracheae the remaining ramose 

 branches originate, and each spreads more particularly to those organs 

 which lie most approximate to it. These branches frequently open 

 into each other, and form stems running contiguously to the intestinal 

 canal, the muscles, and the sexual organs, and whence the delicate 

 branches for these organs originate. 



The number of the branches originating from a main stem, with the 

 exception of the two connecting tubes, is indeed very variable, but we 

 may assume that more branches spread from the tracheae of the thorax 

 than from those of the abdomen. This arises from the greater number 

 of organs existing in the thorax, particularly the number of muscles, 



