The blood-system of a flea is far less complete than that of the lowest vertebrate. The blood is almost colourless. A large contractile heart drives it into the main blood-vessel. There is, however, no closed system of arteries, capillaries, and veins such as the higher animals possess; and the blood circulates in the whole cavity which intervenes between the body-wall and the various internal organs. There is little need for an elaborate system of blood-vessels since the internal tissues are supplied with oxygen by the ramifying air-tubes. Fleas have more of the air-holes called stigmata than any other insects. Each of the three segments of the thorax has a pair, as well as the second to the eighth segments of the abdomen. The spiracles or apertures lie free on the outside of the body. In beetles, and other insects which run through dusty places, they are lodged in the thin membrane between the segments.
The heart of a flea is a very delicate pulsating tube which lies along the back, above the digestive canal and immediately beneath the integument. One may attribute some of the extraordinary strength and vital energy of a flea to the fact that, by the blood-system and the air-system, the tissues of the body are kept richly supplied with oxygen. The blood of a flea is a thin fluid and, of course, without red corpuscles. The blood that is shed when a flea is crushed comes from the stomach and not from the blood-vessels of the insect.
The internal organs of fleas cannot be studied without dissection under a microscope. Dissection is best carried on in a solution of salt and water. Fine needles mounted in penholders are the most handy implements. But the point of even the finest commercial needle that can be bought is too blunt for fine dissections, and it is necessary to sharpen it. This can be done by the help of a rapidly revolving emery wheel, varying the inclination of the needle-point to the wheel, so as to grind off the angles. The flea to be dissected is put in a drop of salt solution, on a slide placed on the stage of the dissecting microscope. In the left hand should be a needle with a blunt conical point, in the right a needle with an oblique point. The antennary groove of the flea should then be transfixed and held firmly by the left-hand needle.
The point of the right-hand needle is then inserted under the edge of the third or fourth abdominal segments. The segments can then be peeled off by a skilful dissector much as we peel off the skin of a shrimp for our tea at the sea-side. The internal organs of the flea then float off in the salt solution; and by using two very fine pointed needles they can be further separated. It is useful to have one needle ready with a hooked end and another fashioned into a minute knife or scalpel.
The most conspicuous of the internal organs will be the stomach and intestine. The salivary glands will be found at the side of the stomach with a certain amount of fat round them. Their extraction is not so difficult as might be supposed. The hooked needle can be used to hook the salivary duct.
The most difficult parts to dissect are the organs connected with the mouth and rostrum. It is best to remove the head and transfix it with the left-hand needle, then to scalp the head by removing the dorsal half of the chitinous carapace. A bold plunge with the right-hand needle will sometimes effect what is desired. A pull on the labium will sometimes bring out the pharynx. It must be confessed that successful dissections are often obtained more by good luck than by skilful management. The use of dilute potash solution facilitates the study of chitinous parts by jellifying the muscles.