534 ORGANS OF RESPIRATION. 



trunks, and other large tracheae, as in many adult dipterous 

 and hymenopterous species and aquatic coleoptera ; such vesi- 

 cular enlargements of the tracheal trunks of insects are seen 

 in the head, the thorax, (144. b.), and especially the anterior 

 part of the abdomen ; they lighten the body in flying forms ; 

 they are covered with minute transparent spots like perfora- 

 tions ; they are not developed in larvae ; and they often give 

 origin to the visceral branches. The common entomoid form 

 of the respiratory organs is seen even in the apterous spe- 

 cies, as the louse, pediculus, where there are seven pairs of 

 lateral stigmata, opening into two longitudinal tracheae, which 

 extend, without dilatations, along the whole body, and com- 

 municate with each other by numerous anastomosing branches* 

 And thus the limited circulation of the blood in insects is 

 compensated for by the extensive ramification and distribu- 

 tion of the respiratory organs through every texture of their 

 body, and the necessary lightness, elasticity, and muscular 

 energy are imparted to these invertebrated winged inhabitants 

 of the air, and hence the high temperature which their body, as 

 has been long known, often acquires. 



In the arachnida, the more extensive circulation of the 

 blood through the body is accompanied, as in annelides, 

 myriapods, and Crustacea, with a more concentrated form of 

 the respiratory organs than occurs in most insects ; and, as 

 in insects, they are always adapted for aerial respiration, 

 whether they belong to aquatic or terrestrial species. Some 

 aquatic forms of arachnida, as some aquatic insects, appear 

 to respire under water by means of the globules of atmo- 

 spheric air which they carry with them into that element, 

 entangled among the hairs of their surface. In the phalan- 

 gium, and other tracheated species, the air- tubes divide into 

 branches from their commencement in the lateral stigmata, 

 as in myriapods, and these branches communicate freely 

 with each other by transverse and longitudinal anastomoses. 

 The stigmata have generally a more ventral aspect in the 

 arachnida, especially in the higher pulmonated forms, as the 

 scorpions and spiders, where no tracheae extend through the 

 body. Some arachnida, as the dysdera and segestria, have 

 been shown by Duges to be at the same time possessed 

 both of ramified tracheae and pulmonary sacs, the two pos- 

 terior of their four stigmata opening into branching air- 



