THE MICROSCOPE. 85 
missure (n, Fig. 1), for two-fifths of a millimeter. In the throat it 
bifureates, each branch being as long as the undivided segment, and 
running on the right and left of the nerve-cord into the prothorax, 
terminates in glands of characteristic structure. These glands are 
CPE 
Tey 
in two sets, one on each side in the antero-inferior region of the pro- 
thorax ; each set consisting of three glands, two of which resemble 
the usual salivary gland. The third gland, which occupies the cen- 
ter of each set, is different, being evenly granular, and staining more 
deeply than the others ; its function being without doubt the secre- 
tion of the poison. Each gland is about one-third of a millimeter 
long, and one-twenty-fifth of a millimeter broad; the three are 
arranged like the leaves of a trefoil ; and each is traversed through- 
out by a fine ductule, the three ductules uniting at the base to form a 
common duct, which is one of the branches of the bifurcated venomo- 
salivary duct. The ductules of the lateral glands of each set 
receive a minute branchlet near the base. Macloskie observed mus- 
cles (m, Fig. 1) apparently inserted on the framework of the reser- 
voir, but Dimmock thinks that the hypopharynx is not furnished 
with muscles. However this may be, the pressure exerted on it by 
the surrounding parts, when the mosquito inserts its piercing appa- 
ratus into the flesh, or through the epidermis of a plant, is sufficient 
to propel the poison through the tubular axis of the hypopharynx 
into the wound. 
Fig. 1. Median section of head : du, venemo-salivary duct with 
its insertion in (hy) hypopharynx; @, cesophagus; cb, cerebrum, below 
which is the cerebellum ; tre, labrum-epipharynx ; m, muscle; n, 
nerve commissure. 
Fig. 2. Venemo-salivary duct, showing its bifurcation and the 
three glands on one of its branches ; pg, poison-gland ; sg, salivary- 
gland. 
Fig. 3. Bifurcation of duct, with nucleated hypodermis. 
