20 Trans. Acad. Sci. of St. Louis. 



symbia for a share of the food of the host; again, as in 

 Oolaelaps oophilus, they feed on the saliva with which 

 ants coat their eggs or their young. As a very gradual 

 step from these symbiotic relationships, we find several 

 facultative or semiparasitic forms, which may be para- 

 sitic for a while, but which can yet maintain themselves 

 without a host. An excellent example of this class is 

 the common chicken mite, Dermanyssus gallinae (Redi) 

 (PI. I, Fig. 2). In favorable circumstances, as in the 

 moist droppings from the roost, they can maintain them- 

 selves almost indefinitely. In the Argasidae we find spe- 

 cies which are very similar to the Dermanyssidae both 

 in habits and structure. They are nocturnal, but may be 

 regarded as obligatory parasites. However, here the 

 parasitic habit has not developed to the extent that it 

 has in the Ixodidae, as has been well established by Nut- 

 tall.® In the true ticks, Ixodidae, the parasite is firmly 

 attached to a single place on its host, hence is a sta- 

 tionary or fixed parasite. However, in most cases it 

 leaves the host to molt. For a sketch of the degenera- 

 tive processes accompanying this progress in the devel- 

 opment of the parasitic habit see the explanation for 

 Plate I. 



In considering the second groups of parasitic Acarina, 

 those most nearly related to the free-living scavengers, 

 we must regard them as representing at least four dif- 

 ferent and phylogenetically distinct lines of descent. The 

 ways in which each separate line arose from scavenger 

 ancestors is practically the same, so only one line will be 

 considered. 



The small family Canestrinidae and many, if not all 

 of the Sarcoptidae, may be regarded as descending from 

 an ancestral type very similar to some of our living Ty- 

 roglyphidae, as the species, Monieziella entomophagus 

 (Laboulbene). This form (see PI. II, Fig. 4), as well 

 as most of the members of the family, is a true scav- 



•Nuttall, G. H. F. On the adaptation of ticks to the habits of their 

 hosts. Parasitology. 4:46-67. 



