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^ 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. 1 1 6) 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. 115 . Dipylidium caninum. 

 When the dog takes up an infested flea or louse, iTt^GrssiTnlR^elu: 

 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 

 116 ' c|^Soid C . an AS 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 



