176 Arthropods as Essential Hosts of Pathogenic Organisms 
114. Dipylidium caninum. 
Rostrum evaginated and 
invaginated. After 
Blanchard. 
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 para¬ 
site, 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. 
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 300(4. 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 contradis¬ 
tinction to the bladder-like cysticercus of many 
other cestodes. It is often referred to in liter¬ 
ature as Cryptocystis trichodectis Villot. 
As many as fifty of the cysticercoids have 
been found in the body cavity of a single flea. 
When the dog takes up an infested flea or louse, 
by biting itself, or when the cat licks them up, the 
larvae quickly develop into tapeworms, reaching sexual maturity in 
about twenty days in the intestine of their host. Puppies and 
kittens are quickly infested when suckling a flea-infested mother, the 
developing worms having been found in the intestines of puppies not 
more than five or six days old. 
Infestation of human beings occurs only 
through accidental ingestion of an infested flea. 
It is natural that such cases should occur largely 
in children, where they may come about in 
some such way as illustrated in the accompany¬ 
ing figures 117 and 118. 
Hymenolepis diminuta, very commonly living in the intestine 
of mice and rats, is also known to occur in man. Its cysticercoid 
develops in the body cavity of a surprising range of meal-infesting 
insects. Grassi and Rovelli (abstract in Ransom, 1904) found it in the 
115. Dipylidium caninum. 
Immature cysticercoid. 
After Grassi and Rovelli. 
110 . 
Dipylidium caninum. 
Cysticercoid. After 
Villet. 
