424 SANITARY ENTOMOLOGY 



SUMMARY 



A good idea of the diversity of life cjcle and of the interrelationship 

 of the tick to its host and its parasite can be obtained by a comparison 

 of the life cycles of HcEinogregarina cams and Babesia canis, both of 

 which pass their cycle of sporogony in the dog tick Rhipicephalus san- 

 guineus and their cycle of schizogony in the dog. Two charts are pre- 

 sented to illustrate the life cycles of these two parasites (figs. 78, 79). 

 It will be noticed that a certain tick, taking up both of these parasites in 

 its nymphal stage from a given dog, would communicate the Haemogre- 

 garina to its adult host, but the Babesia would not be transmitted until the 

 tick's offspring had reached the adult stage, possibly on the third dog host 

 of the offspring. The best way in which to understand how ticks can carry 

 disease organisms is to study the types of life cycles which were worked out 

 by Nuttall, and charts of which are presented. In the first type (fig. 80), 

 found in various species of the genera Argas and Ornithodoros, theic 

 are one larval host, two nymphal hosts, and an indefinite number of adult 

 hosts. Thus, it is apparent that organisms which can be taken up by 

 any one of these stages can be transmitted to quite a number of other 

 hosts by the same tick. 



In type two (fig. 81) there is no larval host but there are five 

 nymphal hosts and an}^ number of adult hosts. This type is found in 

 Ornithodoros moubata and 0. savignyi. It is therefore apparent that 

 the diseases transmitted by these ticks can be conveyed to a number of 

 successive hosts by the same tick. 



The third type (fig. 82), found in the genera Ixodes, Hsemaphysalis, 

 Dermacentor, Rhipicephalus, and Amblyomma, consists of a development 

 with just three hosts, one for the larva, one for the nymph, and one for 

 the adult. Therefore, if the parasite is taken up by the nymph it may 

 be transmitted to the host of the adult, but if the parasite is taken up 

 by the adult, it must either die or be transmitted hereditarily by the off- 

 spring of the tick. 



Type four (fig. 83), found in Rhipicephalus evert si and Hyalomma 

 cegyptium, consists of a development with only two hosts. The larva 

 develops into a nymph on the host and the nymph drops when replete. 

 It reattaches as an adult. The possibilities of transmission are similar 

 to those in type three, but tend more toward hereditary transmission. 



Type five (fig. 84), represented by the genus Boophilus, has only 

 one host. The larva attaches and goes through its entire transformation 

 on the host. It is, therefore, apparent that any organism transmitted 

 by these ticks must be transmitted hereditarily. 



Type six (fig. 85) is similar in that there is but one host. It is repre- 

 sented by Ornithodoros megnini, which is on the host during its larval 



