484 THE ANIMAL PARASITES OF MAN 



amount to six, of which the two front pairs, the chelicerae and the 

 pedipalpi, are attached to the head region and the four remaining 

 pairs to the thoracic region. 1 The abdomen in the adult condition 

 has no appendages. The Arachnoids are air-breathers, and for this 

 purpose are either provided with tracheae or with so-called lung-sacs, 

 or they breathe through the surface of the body. Some aquatic forms 

 breathe by gills. 



There are eight or ten orders of Arachnoids, 2 of which, however, 

 only two, the Acarina and the Linguatulida, have to be considered 

 here. 3 



Order. Acarina (Mites). 



Small Arachnoids, the three parts of the body of which are, as a rule, coalesced ; 

 it is only rarely that a faint line indicates the division between a cephalothorax and 

 abdomen. The two appendages on the head are designed for biting or puncturing 

 and sucking, and vary according to their use. The chelicerae 4 are fang-like jaws 

 or puncturing bristles forming a kind of rostrum, the pedipalpi are claw-like or shear- 

 shaped, or form a suctorial proboscis. 5 The four pairs of legs are usually well 

 developed, more rarely they are rudimentary or have partly vanished ; many 

 parasitic forms are provided with pedunculated suckers [ambulacra F. V. T.]. 

 Respiratory organs (tracheal tufts) may be present or absent. The nervous system 

 is reduced to a minimum, eyes are usually lacking. The intestine, situated in the 

 central part, generally has three blind appendages ; the anus is situated on the venter 

 above the posterior end. Sexes separated ; nearly all the species deposit eggs, from 

 which six-legged larvae hatch. The Acarina live either free in the water or in moist 

 soil, or they are parasitic on plants and animals. 6 



1 [The true character of the Arachnoidea is the presence of four pairs of ambulatory 

 appendages. This number is reduced to two pairs in the gall-making Phytoptida, and they 

 differ from all other Arthropoda in having no antennae. F. V. T.] 



2 Twelve orders are now recognized, as follows : Pentastomida or Linguatulids ; Tardi- 

 grada or bear-animalcules ; Phalangida or harvest-men ; Acarina or ticks and mites ; 

 Palpigradi ; Solifugce; Pseudoscorpionidea or book mites; Pedipalpi or false scorpions; 

 Scorpionidea or true scorpions ; Araneida or spiders ; Xiphosura or king crabs ; and Pycnogonida> 

 marine Arachnoids. 



8 Chelifer cancroides has also been observed as a pseudoparasite in man (Arnault de 

 Very, S., " Pseudopar. du. CheL cancr. chez 1'homme," Compt. rend. Soc. de Biol., 1901, 

 liii, p. 105). 



4 [The chelicerse are sometimes regarded as modified antennae, but it is more natural to 

 regard them as the morphological equivalent of the mandibles of ffexapoda.Y. V. T.] 



5 [The pedipalpi, or second pair of jaws, consist of a stout basal segment and a palp, which 

 may have the appearance of a leg in Arachnida ; this may end with or without a claw, or with 

 a chela (scorpions) ; they may also form a tube enclosing the styliform chelicerge (mites). 

 F. V. T.] 



6 [Acarina are also found living upon trees, feeding upon other Arthropods and also upon 

 spores of lichen and fungi [Oribatida or beetle mites) ; they also swarm indoors amongst stores 

 and provisions (Tyroglyphidce and Glyciphagi, household, sugar and cheese mites). This 

 order is very important, as many are parasites upon man, his domestic animals and his cultivated 

 plants, and attack his provisions and stores. Some live on blood, and in some of the ticks 

 distribute various protozoal and other blood parasites and germs. F. V. T.] 



