Ewing — Significance of Parasitism in Acarina. 67 



EXPLANATION OF PHYLOGENETIC TREE. 



In the diagram, or phylogenetic tree, given on the opposite page 

 the writer has shown by the use of a series of symbols (squares, 

 circles, dashes, etc.) much more than is generally done in such trees. 

 This is in order to give the habits of these arachnids as they have 

 been traced in this phylogeny. Thus, although according to this 

 diagram we have living mites with eight different kinds of feeding 

 habits, they all have descended from predaceous forms. 



As we pass in position from the top of the page to the bottom, we 

 get a regular decline in the state of evolution. Forms with many 

 specialized parts or organs and with highly developed functions are 

 considered highly evolved; those with many similar parts and with 

 these parts not highly specialized, or those with few parts and with 

 functions poorly developed are considered as being less highly evolved. 



EXPLANATION OF ILLUSTRATIONS. 



Plate I. — Steps in the Origin of Parasitism from Predaceous Types. 

 — Fig. 1. Gamasus dentatus Ewing, dorsal view. This species is 

 typical of the free living forms of Gamasidae. They are predaceous 

 in habit and evidently are very similar to the ancestral predaceous 

 type or types from which the Dermanyssidae, as well as the Argasidae 

 and Ixodidae, arose. — Fig. 2. Dermanyssus gallinae (Redi), female, 

 dorsal view. This is the notorious chicken mite of domestic poultry. 

 As yet it may be considered a facultative parasite as it can maintain 

 itself indefinitely away from its host if suitable conditions are at 

 hand. In this stage of parasitism, which is only a step toward the 

 final one, degeneration and adaptation are well exhibited as may be 

 seen by comparing this figure with Fig. 1. The legs have become 

 somewhat shortened, the palpi are much smaller and the body is 

 much depressed in form, while the chelicerae are modified into long 

 piercing organs. — Fig. 3. Boopliilus annulatus Say. The well known 

 Texas Fever Tick. In the Ixodidae the parasitic habit has become 

 permanent, the parasite, however, leaves its host in most cases in 

 order to molt. Degeneration and specialization has continued as is 

 seen in the great reduction of the size of the legs and mouth-parts, 

 in the flattened form of the body, and in the enormous development 

 of the hypostoma which is provided with recurved teeth for the pur- 

 pose of clinging to a host. 



Plate II. — Steps in the Origin of Parasitism from Scavenger Types. 

 — Fig. 4. Monieziella entomophagus (LaboulbSne). This mite belongs 

 to a small family of scavenger mites. It does not live upon live 

 insects, as the name suggests, and as has been long supposed by most 

 entomologists, but only chews and sucks the juices of dead insects 

 and other decaying organic materials. It evidently is very similar to 

 the ancestral type from which a very large group of our most im- 



