MITES AND TICKS 183 



ist, and in this chapter only the briefest outHne of the biology of the 

 group will be attempted. 



Classification and distribution 



The Acari do not represent a natural group, but comprise 

 several heterogeneous and distinct phylogenetic lines. They are 

 mostly of small size and possess four, six or eight legs: usually the 

 larval stages have three pairs, the nymphal and adult four pairs of 

 limbs. The shape of the body varies considerably and may be 

 elongated and worm-like, short, rounded, elliptical or spherical. 

 The more primitive forms show traces of segmentation which dis- 

 appear completely in the more advanced groups. Six sub-orders 

 are recognised by Andre {in Grasse, 1949)* and these are separated 

 largely according to the position of the spiracles. The Notostig- 

 mata is a small group of primitive mites in which the segmentation 

 of the body is manifest. The metapodosoma consists of two 

 somites, the opisthosoma of nine. These animals resemble harvest- 

 spiders in general form but are brightly coloured and exhibit a 

 combination of gold, blue and violet hues unknown elsewhere in 

 the order. They are all referred to a single genus Opilioacarus, the 

 species of which are found free-living under stones and are widely 

 distributed in Europe, North Africa and South America. 



The sub-order Holothyroidea again contains but a single genus 

 of wide distribution found in the Seychelles, Mauritius, Australia, 

 New Guinea and Ceylon. A few species of Holothyrus found in 

 Papua are of interest because their size, which may reach 7 mm 

 makes them the largest of all the mites, other than ticks. Their 

 segmentation is hidden by a strongly chitinised plate which covers 

 the upper surface of the body. The Notostigmata and Holothy- 

 roidea are both included in the sub-order Onychopalpida by 

 Baker and Wharton (1952). 



The third sub-order, the Parasitiformes, contains a vast assem- 

 blage of genera and species both free-living and parasitic on other 

 animals and plants. They fall into two distinct groups, the Meso- 

 stigmata containing the super-families Gamasides and Uropodina 

 and the Ixodoidea or ticks, which in turn are subdivided into three 

 super-families, the Ixodei, the Nuttalliellei and the Argasides. 

 From the point of view of the biologist the Mesostigmata are the 



