MEDICAL ENTOMOLOGY 



Suborder. 



ASTIGMATA. 



Suborder. VERMIFORMIA. 



ARACHNIDA INSERTIS SEDIS. 



. Trombidiidae, Hydrachnidae, 

 Holacaridae, and the Bdellidae. 



Degenerate acari with soft skins 

 and no tracheae ; they are par- 

 asitic and approach the PRO- 

 STlGMATAin the development of 

 the integumental sclerites. This 

 order contains the Sarcoptidae. 



Degenerate acari without tracheae 

 and eyes ; the posterior portion 

 of the body is protruded into an 

 annulated tail ; the four pairs 

 of legs are short, and three- 

 jointed. This order contains 

 the Demodicidae. 



PENTASTOMIDA show many dis- 

 tinct arthropod characters ; they 

 will be dealt with in Chapter x, 

 Section 1. 



It will be noticed that the blood-sucking forms are distributed some- 

 what irregularly throughout the ARTHROPODA, and are not confined to any 

 one order, though some orders, such as the Siphonaptera, are exclusively 

 blood-sucking and parasitic. The habit is in itself no indication of 

 relationship, as is shown by the fact that it may be present in the female 

 and absent in the male. Mouth parts which have become adapted for 

 the purpose of piercing the skin and sucking up fluid nourishment are, in 

 fact, formed from the same elements as those which are adapted for the 

 ingestion of solid food, or fluid obtained from other sources. The mouth 

 appendages, that is, the mandibles and first and second maxillae, are of 

 such a type that they can, without any very great divergence from their 

 ancestral form, be adapted to serve such a purpose. The form of ap- 

 paratus which has evolved in the different families necessarily shows a 

 good deal of variety, but it can usually be resolved into portions corre- 

 sponding with the mouth appendages of non-blood-sucking forms. In the 

 ticks, for instance, the mandibles or chelicerae make the wound, while 

 the sucking tube up which blood is drawn is formed by their apposi- 

 tion to the hypostome. In the bugs the mandibles and maxillae 

 are of great length, the former being so apposed as to form a 

 channel with a circular lumen, while the maxillae are armed with cutting 



