INTERNAL ANATOMY OF INSECTS. 9S 
by retrograde movements of the currents, or by oscilla- 
tory motions of the blood-globules. 
In proportion as the wings are developed, the circula- 
tion in the laminae diminishes, and ultimately ceases, pre- 
paratory to the detachment of the laminae themselves. 
At the same time, however, it presents itself under a new 
form in the wings. In these the excurrent or arterial 
stream takes its course along the inner margin of the 
wing, and the recurrent or venous returning along the 
outer; whilst, occasionally, other transverse currents take 
their course through the net-work of the wing from its 
inner to its outer margin. As the wings are further 
developed, the circulation in them, like that in the cau- 
dal laminae, gradually becomes weaker and ultimately 
ceases a . 
The next observations were made on the transparent 
larva of a neuropterous insect (probably a Semblis or 
Sialis), in which the pulsations of the dorsal vessel were 
distinctly seen at its posterior extremity, from which they 
were propagated towards the anterior ; these two divi- 
sions of that vessel appearing to bear to each other the re- 
lation of a heart and aorta. There were no traces of other 
vessels, though regular and rapid currents of blood-glo- 
bules, exterior to the tracheae, proceeded from the head 
towards the posterior extremity of the body, where each 
of these currents entered the heart, which again propelled 
its contents with accelerated velocity through the anterior 
part of the dorsal vessel towards the head. The lateral 
currents also were accelerated upon each contraction of 
the heart, proving that they must communicate with the 
dorsal vessel at the anterior part of the body, though the 
a Introd. to Covtp. Anat. ii. 393 — . Engl. Trans. 
