In some cases, the sexual cycle of the parasite is undergone in the arthropod, which then serves as the definitive or primary host. In other cases, it is the asexual stage of the parasite which is undergone, and the arthropod then acts as the intermediate host. This distinction is often overlooked and all the cases incorrectly referred to as those in which the insect or other arthropod acts as intermediate host.
We have already emphasized that this is the most important way in which insects may transmit disease, for without them the particular organisms concerned could never complete their development. Exterminate the arthropod host and the life cycle of the parasite is broken, the disease is exterminated.
As the phenomenon of alternation of generations, as exhibited by many of the parasitic protozoa, is a complicated one and usually new to the student, we shall first take up some of the grosser cases illustrated by certain parasitic worms. There is the additional reason that these were the first cases known of arthropod transmission of pathogenic organisms.
Insects as Intermediate Hosts of Tapeworms
A number of tapeworms are known to undergo their sexual stage in an insect or other arthropod. Of these at least two are occasional parasites of man.
Dipylidium caninum (figs. [113] and [114]), more generally known as Taenia cucumerina or T. elliptica, is the commonest intestinal parasite of pet dogs and cats. It is occasionally found as a human parasite, 70 per cent of the cases reported being in young children.
