188 Dr. T. Williams on the Mechanism of Aquatic 



under some form in the Myriapod and the Insect. They consti- 

 tute a conspicuous class of distributive vessels in the Anne- 

 lids. Mr. Newport docs not suspect even their existence in the 

 Myriapod and the Insect. He replaces them by his incompre- 

 hensible systemic arteries. In the pulmonary Arachnids a great 

 venous abdominal trunk (fig. 3, e, I) is superadded to the system 

 of the circulation. It conveys forwards the blood (according to 

 Mr. Newport) from the caudal region. This again is undoubt- 

 edly an error. In this vessel in the Arachnid^ as in the Insect, 

 the course af the blood is backwards ; it distributes it over the 

 branchiae, from which it is returned by the pneumo -cardiac 

 channels to the heart again to repeat the same course. 



The corresponding central parts of the tracheary apparatus 

 require but few preparatory words : — 



In the Myriapods the large trachese communicate externally 

 with the spiracles. In the Scolopendrida, e.g. the LithobiuSy 

 they exist on alternate rings to the number of eight or ten. The 

 tracheae proceed thence in longitudinal trunks to be distributed 

 over every part of the body. 



In Insects the spiracles are usually nine in number on eacli 

 side; each spiracle consists of a horny ring of an oval form, 

 within which is a valve formed of a series of converging fibres, 

 and which opens perpendicularly on its long axis guarding the 

 external entrance*. In the perfect Insect the spiracles of the 

 abdomen are small, those of the larva large. In the latter, abdo- 

 minal respiration is most active ; in the former, the thoracic is 

 predominant. In Insects as in Myriapods, the tracheae, arising 

 at the spiracles, are distributed over every part of the body 

 (Plate X. fig. 8). The tracheae of all larvae are simply tubular,, 

 those of all volant perfect insects are dilated at vaiious parts 

 i?nto vesicles. 



. In some tribes, as in most of the Hymenoptera, Lepidoptera, 

 dnd Diptera, these sacs are present in almost every species and 

 occupy a large portion of the interior of the body, more espe- 

 cially of the abdominal region. They exist only in the volant 

 species of the Coleoptera. They are present in the winged Cara- 

 bidae, but not in the apterous. The tracheae, in those species 

 of Orthoptera which are merely saltatorial in habits, never dilate 

 into vesicles. M. Umile Blanchard declares that the substance 

 of the walls of these vesicles is channelled into plexiform passages 

 for the blood. This is most certainly incorrect. These sacs 

 have no reference to the respiratory process ; they subserve only 

 a mechanical use ; without them the insect could not fly. They 

 exist in the male of the common Glow-worm, but not in the 

 female (Newport). 



* Art. Insecta, Cyclop. Anat, and Phys. 



