The Poison-Apparatus of the Mosquito. 887 
The glands are in two sets, one on each side in the antero-inferior 
region of the prothorax. Each set consists of three glands, two of 
which are of the usual aspect of salivary glands, resembling in 
structure, but not proportionately as long as, the single salivary 
gland on each side in the prothorax of the house-fly. The third 
gland, that occupying the centre of each set, is different, being 
evenly granular, and staining more deeply than the others; its 
function being without doubt the secretion of the poison. Each 
gland is about one-third of a millimetre long, and one twenty-fifth 
of a millimetre broad ; the three are arranged like the leaves of a 
trefoil; and each is traversed throughout by a fine ductule, the 
three ductules uniting at the base to form a common duct, which is 
like a pedicel of the trefoil and is one of the branches of the bifur- 
cated venomo-salivary duct. The ductules of the lateral glands of 
each set receive a minute branchlet near the base. Thus there are 
six glands, three on each side, two of them poisonous and four 
EXPLANATION OF FIGURES.—Fig. 1. Median section of head, show. 
ing (du) the venomo-salivary duct, with its insertion in (hy) the hypo- 
pharynx: cb, cerebrym; below this is the cerebellum, and the pumping 
enlargement of (œ) the cesophagus : (Jr. e.), base of labrum-epipharynx; 
(m) muscle; (n) nerve-commissure. Other parts removed. 
Fig. 2. The venomo-salivary duct, showing its bifurcation, and the 
three glands on one of its branches: (pg) poison-gland; (sg) marks 
the upper of the two salivary glands. 
Fig. 3. The bifurcation of the duct, with its nucleated hypodermis. 
