338 THE ARACHNOIDS 



The chelicerae are needle like with inconspicuous palps and the front and rear 

 pairs of legs are widely separated. The gravid female is like a ball and is about 

 looofA in diameter. 



They live on wheat and may be found in wheat straw, which, if handled, may be 

 followed by a severe skin eruption with an irregular fever. 



Ixodidae 



This family of the Arachnoidea is one of great medical interest and 

 of growing importance. It has recently been proposed to raise the 

 ticks to a superfamily, Ixodoidea and to divide it into the families 

 Argasidae and Ixodidae. 



While only proven the intermediary hosts in the case of the organism of African 

 tick fever and the recently discovered cause of spotted fever of the Rocky Moun- 

 tains, there is considerable speculation as to the possibility of blackwater fever being 

 due to a Babesia (Piro plasma}. Piroplasmata of animals seem to be invariably 

 transmitted by ticks. 



Very important diseases due to these small pear-shaped organisms within red 

 cells are known for various animals, the best known being that of cattle in Texas 

 and known as Texas fever. Other piroplasmata diseases are Rhodesian fever 

 (cattle), heart water (sheep), and malignant jaundice of dogs. In these diseases 

 there are pathological features which resemble blackwater fever of man. 



It is of interest to note that it was with the transmission of Texas 

 fever through an intermediate host (the tick) that Smith and Kilborne 

 (1889-1893) established the zoological principle of transmission of 

 disease through arthropod intermediary hosts. This led up to the 

 work on malaria, yellow fever, etc. 



Ticks differ from insects in having four pairs of legs, only two pairs of mouth 

 parts, and no antennae. They differ from other acarines in having a median probe- 

 shaped puncturing organ, the hypostome, which is beset with numerous teeth 

 projecting backward, and in possessing stigmal plates. The head, or capitulum, 

 or rostrum, is the part which projects anteriorly from the body. This carries the 

 piercing parts which are the single hypostome or dart and a pair of piercing chitinous 

 structures, the chelicerae which lie above the hypostome. As a sheath for these 

 delicate biting parts we have a segmented pair of palpi or pedipalps. The mouth 

 is a slit between the chelicerae and hypostome. 



Two depressed pitted areas on the dorsal surface of the capitulum in the adult 

 female are known as porose areas. Very important structures are the stigmal 

 plates. These are striking mosaic-like areas which are located just posterior to 

 each hind leg in the Ixodinae and between the third and fourth legs in the Argasinae. 

 As the greatest confusion exists as to the classification of ticks, Dr. Charles W. 

 Stiles has now in hand a system of classifying ticks according to the appearance 

 of these plates as seen under the high power of a microscope. There is great varia- 



