ANATOMY AND PHYSIOLOGY 127 
ity to forma simus, in which the blood flows backward, bathing 
the ventral nerve cord as it goes. This ventral sinus supple- 
ments the heart in a minor way, as do also the local pulsatory 
sacs which have been discovered in the legs of aquatic Hemip- 
tera and the head of Orthoptera. 
Blood.—The blood, or hemolymph, of an insect consists 
chiefly of a watery fluid, or plasma, which contains corpuscles, 
or leucocytes. Though usually colorless, the plasma is some- 
times yellow (Coccinellide, Meloidz), often greenish in her- 
bivorous insects from the presence of chlorophyll, and some- 
times of other colors; often the blood owes its hue to yellow 
or red drops of fat on the surface of the blood corpuscles 
(Fig. 161). . 
Leucocytes.—The corpuscles, or leucocytes, are minute 
nucleated cells, 6 to 30 » in diameter, variable in form even 
in the same species but commonly (Fig. 161) round, oval or 
ovate in profile, though often disk-shaped, elongate or amce- 
sboid in form. 
Function of the Blood.—The blood of insects contains 
many substances, including egg albumin, globulin, fibrin, iron, 
potassium and sodium (Mayer), and especially such a large 
amount of fatty material that its principal function is probably 
one of nutrition; the blood of an insect contains no red cor- 
puscles and has little or nothing to do with the aeration 
of tissues, that function being relegated to the tracheal 
system. 
Circulation.—The course of the circulation is evident in 
transparent aquatic nymphs or larvee. In odonate or ephe- 
merid nymphs, currents of blood may be seen (Fig. 162) flow- 
ing through the spaces between muscles, trachez, nerves, etc., 
and bathing all the tissues; separate outgoing and incoming 
streams may be distinguished in the antennz and legs; the 
returning blood flows along the sides of the body and through 
the ventral sinus and the pericardial chamber, eventually to 
enter the lateral ostia of the dorsal vessel. A circulation of 
blood occurs in the wings of freshly emerged Odonata, Ephe- 
