a handbook of the mosquitoes of north america - Systematic Catalog ...

26 MOSQUITOES OF NORTH AMERICA
inous plates. are provided with powerful muscles attached to
the head capsule so that the capacity of the pharynx can be
greatly increased by their contraction. The relaxation of the
muscles allows the walls to contract owing to the great elasticity
of the chitinous plates. By this means the mosquito can suck
up liquids into its pharynx and as the muscles relax the con-
tained liquids are forced onward to the esophagus. The eso-
phagus is a thin-walled tubed extending to the esophageal valve.
Near its posterior end open three large diverticula or sacs.
The ventral one is the largest and extends backward to the
sixth or seventh abdominal segment; the other two dorso-lat-
era1 sacs are much smaller and are contained within the thorax.
The function of these sacs is not fully known though they
usually contain gas bubbles and act as food reservoirs. The
esophageal valve or proventriculus serves as the connecting
link to the mid-intestine and is surrounded by a very thick
sphincter muscl’e.
The mid-intes,tine or mid-gut is the only part of the food
canal that is not lined with chitin and it is believed that only
in this part can food absorption take place. It is a straight
tubular organ extending from about the middle of the thorax
to near the distal third of the abdomen. The posterior por-
tion is somewhat expanded and is generally referred to as the
stomach. The stomach is capable of great distension allowing
the insect to obtain large quantities of blo,od. It is also in
this portion that the malarial parasites develop. The stomach
connects directly with the hind intestine and at the point of
juncture there arise five malpighian tubules. The hind intestine
extends to the anal opening. It varies somewhat in size and
shape as it passes backward so that the names ileum, colon
and rectum have been applied to separate parts. These are
vertebrate terms and have probably no, significance here. The
malpighian tubules are elongate narrow tubes composed of
large secretory cells.
The salivary glands are important structures. These con-
sist of three small glands situated on each side of the thorax