A flea like every other animal must feed and breathe, which leads to a consideration of the internal organs of digestion and respiration. The digestive canal is a slender tube which connects the mouth and the anus, and which is less convoluted and much straighter than in the higher vertebrates. [Fig. 6] will show the relative positions of the various parts, namely, the mouth, pharynx, gullet, gizzard, stomach, and rectum. Connected with the digestive canal are certain glands and organs of excretion. The alimentary tube itself passes through the middle of the flea’s body, and is kept in that position partly by muscles and partly by the numerous branching air-tubes through which the insect breathes. Above it lies the heart, and beneath it the nervous cord or chain of ganglia.
Fig. 6. Diagram of the alimentary canal of a flea. At the top is shown the orifice of the mouth, leading into the pharynx. Next comes the short gullet. The gizzard is the smaller organ immediately before the stomach. At the base of the stomach are four vermiform tubes, which are the Malpighian tubules. From the base of the stomach issues the intestine, which leads to the rectum, where the six rectal glands are shown.
The mouth of a flea, as of any other insect, is merely an orifice which forms the opening into the alimentary canal. Around the orifice are the various mouth-parts which convey blood to the mouth, but these, the reader will doubtless remember, are the modified limbs or appendages of the segments that compose the flea’s head. The mouth, then, gives access to the digestive canal. The first part nearest the mouth is the pharynx which merges gradually into the gullet. Here is placed the pharyngeal pump which is provided with a sucking apparatus. Muscles attached to the dorsal part of the so-called aspiratory pharynx cause it to expand and contract, owing to the elastic reaction of its walls. The operating muscles, which do this, are in the head of the flea. When these pharyngeal muscles contract and relax in regular sequence, a rhythmic action of the pharynx itself ensues and a steady stream of blood is forced or drawn from the mouth stomachwards. In a light coloured flea, under a powerful lens, this action may be watched in the living insect.
Behind the pharynx comes the gullet, which leads down to the gizzard. It is perhaps needless to add that this organ, neither in appearance nor in use, bears any resemblance to the gizzard of a bird, which grinds hard food. The food of the adult flea consists solely of liquid blood.
The organ called gizzard in the flea, for want of a better name, is, however, remarkable. Its function is not quite certainly known. It is a bulbous expansion in the front of the stomach and situated at the junction of the stomach and the gullet. It contains a multitude of chitinous finger-like processes tapering towards their extremities. From their general arrangement the complete collection of processes would act as an effective sort of valve and prevent the return of the fluids from the stomach. It seems most probable that this is their function. During the life of the flea the stomach is constantly churning its contents. Some valvular arrangement between the stomach and the pharynx would seem to be essential; the pharynx is normally collapsed, as the reader may remember, and its walls are drawn apart by muscles attached to its exterior. When the pharynx is full of blood the muscles relax, the walls collapse like elastic, and the blood is forced into the stomach. In many cases a flea will feed when the stomach is already tensely full of blood; and some sort of valve is therefore needed to prevent regurgitation into the pharynx when the pharyngeal muscles contract and the walls of the pharynx itself are drawn asunder.
This valvular arrangement at the anterior end of the flea’s stomach has been minutely studied in connection with recent plague investigations, because there was a theory that fleas carried infection by vomiting the septicæmic blood from their stomachs and so transferred the plague bacillus to the puncture which they made in the skin.
But an experiment, which has been tried several times, seems to show that the supposed valve is effective. The stomach of a flea which had recently fed was dissected out intact. As long a portion of rectum as possible was left attached at the hinder end. The gullet having been severed, well in front of the valve, pressure was applied with a blunt tool with the object of forcing the blood through the gullet. The hind aperture of the stomach was, at the same time, closed by pinching up the rectum. The result was that, in no instance, was it possible to force blood through the passage which leads into the gullet. Yet sufficient pressure was applied to burst the stomach.