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. In 1869, Melnikoff found in a dog louse, _Trichodectes canis_, some peculiar bodies which Leuckart identified as the larval form of this tapeworm. The worm is, however, much more common in dogs and cats than is the skin parasite, and hence it appears that the _Trichodectes_ could not be the only intermediate host. In 1888, Grassi found that it could also develop in the cat and dog fleas, _Ctenocephalus felis_ and _C. canis_, and in the human flea, _Pulex irritans_. [Illustration: 114. Dipylidium caninum. Rostrum evaginated and invaginated. After Blanchard.] [Illustration: 115. Dipylidium caninum. Immature cysticercoid. After Grassi and Rovelli.] The eggs, scattered among the hairs of the dog or cat, are ingested by the insect host and in its body cavity they develop into pyriform bodies, about 300µ in length, almost entirely destitute of a bladder, but in the immature stage provided with a caudal appendage (fig. 115). Within the pear-shaped body (fig. 116) are the invaginated head and suckers of the future tapeworm. This larval form is known as a cysticercoid, in contradistinction to the bladder-like cysticercus of many other cestodes. It is often referred to in literature as _Cryptocystis trichodectis_ Villot.