THE ACARINA. 339 



glands are divisible into five different kinds (aciniform, am- 

 pullate, aggregate, tubuliform, and tuberous), and their ducts 

 ultimately enter the six prominent arachnidial mammillce, 

 which, in this species, project from the hinder end of the 

 abdomen. The superior and inferior mammillae are three- 

 jointed, the middle one is two-jointed. Their terminal faces 

 are truncated, forming an area beset with the minute arach- 

 nidial papilloe by which the secretion of the glands is poured 

 out. 



The males are smaller than the females, and their ap- 

 proaches to the latter are made with extreme caution, as 

 they run the risk of being devoured ; extending their pedi- 

 palps, they deposit the spermatophores in the female genital 

 aperture, and betake themselves to flight. 



The Aranelna are oviparous, but the development of the 

 embryo takes place as in the Arthrogastra, and there is no 

 metamorphosis. 1 



The Acarina. — In the Mites and Ticks, the hinder so- 

 mites are, as in the Spiders, distinctly separated from one an- 

 other, but they are not separated by any constriction from 

 the anterior somites. 



The bases of the chelicerae, and of the pedipalpi, coalesce 

 with the labrum, and give rise to a suctorial rostrum (Fig. 

 90). 



There are usually several gastric caeca, but no distinct 

 liver. Salivary glands occur in some, and Malpighian caeca 

 are occasionally found. No heart has yet been discovered. 

 Special respiratory organs are sometimes wanting (e. g., Sar- 

 coptes) ; when present, they are tracheal tubes, springing 

 brush- wise from a common trunk which opens by a stigma. 

 The stigmata are usually two, sometimes anterior and some- 

 times posterior in position. The ganglia of the nervous sys- 

 tem are concentrated round the gullet, as in the Sj)iders ; and 

 the reproductive aperture is situated far forward, sometimes 

 close to the rostrum. 



The greater number of the Acarina are parasites upon 

 other animals or upon plants. 



Most are oviparous, but the Orihatidce are viviparous. 

 The course of the development of the embryo is the same as 

 in the Spiders. The young, when born, are frequently pro- 



1 Claparede, " Kecherches sur l'Evolution des Araignees," 1862. Also 

 Balbiam, "Ann. des Sc. Nat.," 1873. 



