1J0 
ANATOMY AND PHYSIOLOGY 
usually straightest in larvae, (particularly in cater- 
pillars,) and in the imago it curves downwards 
between the thorax and abdomen, to enable it to pass 
through the narrow isthmus by which these parts 
are united. On close examination it is found that 
the coats of this vessel are composed of two mem- 
branes, the exterior of which is dense and muscular. 
Such at least appeared their composition to Strauss, 
but Mr. Newport detected a third membrane of an 
extremely thin and delicate texture. 
The greater portion of the vessel contained in the 
abdomen, consists of a consecutive series of chambers 
or cells, separated from each other either by single 
or double valvules, which permit the transmission of 
the blood from behind forwards, or in the direction 
of the head, but prevent its retrograde motion. fe A 
little behind these valvules, on both sides of each 
chamber, there is a transverse opening, likewise 
having a semilunar valvule internally, which allows 
the blood to enter the organ and opposes its exit. 
The number of the cells varies according to the 
species, or rather the families. M. Strauss found 
eight in the Melolontha vulgaris, M. Burmeister five 
in the larva of Calosoma Sycophanta, while M. J. 
Muller discovered onl} r one in Phasma ; but in this he 
was probably mistaken. However this may be, the 
organization in question explains at once the pro- 
gress of the blood in the dorsal vessel, and the reason 
of it being always filled. For when the hinder cell, 
which is usually shorter than the others, and which 
we shall suppose full of blood, contracts, the fluid, 
pressed on all sides by the containing vessel, is forced 
