ACARINA. ACARI. MITES. TICKS. 



These belong to the class Arachnida which includes, spiders 

 and scorpions, as well as mites, and ticks. As common characters 

 it may be noted that the head and thorax have become confluent 

 (cephalo thorax), that the mature forms have four pairs of legs, 

 and are devoid of antennae. 



Acarus, Mite. These have head, thorax and abdomen con- 

 fluent, so as to form one continuous mass. Some species how- 

 ever show a groove between the head and thorax and others also 

 between the thorax and abdomen. Many have the chitinous sur- 

 face marked by fine transverse striae and growing a few hairs. 

 The acari are mostly very small, almost microscopic. They are 

 air breathing by 2 stigmata, have the sexes in different individuals 

 and are oviparous (one family is viviparous). 



The successive stages of development may be thus stated : 



1. The ovum. 



2. The larva with six X&^s^hexapod'); has two abdominal bristles 

 which may represent the missing pair of legs ; .sexually imma- 

 ture ; moults 2 or 3 times. 



3. Nym.pha with eight legs {octopod); asexual ; moults once. 



4. Sexually m.ature m,ale and fem,ale. 



5. Ovigerous female. 



The table on page 112 gives prominent distinctive features of the 

 different families of acarina, the parasitic families being given 

 in black-faced type. 



HABITS AND PATHOGENESIS. 



The ticks are temporary parasites, many attacking all or 

 nearly all terrestrial vertebrata and so far as the nymphse 

 and males are concerned parasitism is not obligatory and 

 they show often very little preference for one genus over 

 another. The ovigerous females on the other hand are com- 

 pulsory parasites and usually show a very marked preference for 

 a given genus or species, so that the absence of that genus 

 is often equivalent to the extinction of that race of ixode. 



They are found especially in cultivated lands, wooded or cov- 

 ered with brush or tall, coarse vegetation, where they may be 

 seen hanging to the leaves by their first pair of feet, while the 



III 



