184 MITES AND TICKS 



more interesting, since the range of habitat and mode of life is 

 more varied. The group seems to have become adapted to different 

 habitats with little resulting change in fundamental structure. 



The ticks are the most familiar of the mites, not only on account 

 of their large size, but because they are of medical and veterinary 

 importance. The majority of species are included in the family 

 Ixodidae in which a scutum or shield is always present. The sexes 

 are distinct: the males feed little or not at all, but the females and 

 immature stages imbibe large quantities of blood so that the scutum 

 becomes a mere patch on the anterior portion of their greatly dis- 

 tended bodies. They take several days to gorge themselves, and 

 each stage feeds only once. 



The Argasidae, on the other hand, are ticks with leathery inte- 

 gument, no portion being specially chitinised to form a scutum. 

 When gorged there is no extravagant increase in size as in female 

 Ixodidae. 



The fourth sub-order is the Thrombidiformes or Trombidi- 

 formes which includes a very diverse group of mites probably 

 derived from more than one ancestral type. A considerable degree 

 of segmentation is retained in many species of the super-family 

 Tarsonemini which contains a number of insect parasites. The 

 super-family Prostigmata comprises a group known as the Eleu- 

 therengona in which are found a number of plant-feeding families, 

 of which the most important economically are the 'red-spider' 

 mites or Tetranychidae, the universal follicle-mites Demodex spp., 

 the familiar Bdellidae or 'snout-mites' and the salt water Hala- 

 caridae. 



The Parasitengona is the group of the Thrombidiformes which 

 contains the fresh water mites, sometimes known as Hydracarina. 

 These belong to a large number of families such as the Hydra- 

 chnidae, Limnocharidae, Eylaidae, Limnesiidae and Arrenuridae, 

 etc. They are related to the irritating harvest-mites, chiggers or 

 Thrombidiidae which are becoming increasingly important medi- 

 cally as they have been shown to transmit many forms of scrub- 

 typhus (Audy and Harrison, 1951). 



The fifth sub-order, the Sarcoptiformes, is probably the most 

 specialised and external segmentation is reduced to a minimum. It 

 contains two large and successful groups: the Acaridiae which in- 



