282 THE ARACHNOIDS 



Respiration takes place through the medium of gills in the Crustacea 

 and by tracheal tubes in the Myriapoda, Arachnoidea, and Insecta. 



The Arachnoidea have no antennae while the Myriapoda and Insecta 

 have a single pair of antennae, the former having numerous pairs of 

 legs or jointed appendages while the latter have only three pairs of legs. 

 The Arthropoda have segmented bodies, but they differ from the worms 

 in having jointed appendages for the purpose of taking in food and 

 moving from place to place. They also have an exoskeleton which is 

 more or less unyielding from the deposit of chitin in the cuticle. This 

 cuticle is not a true skin but only a secretion of the epidermis. . 



Within this external skeleton we have a dorsal digestive system and 

 a ventral nervous system. 



THE ARACHNOIDEA. 



The Arachnoidea differ from the Insecta in having the head and 

 thorax fused together. They also have four pairs of ambulatory appen- 

 dages, while the insects only have three pairs. The Arachnoidea never 

 have compound eyes 'these when present being simple. Of the two 

 orders of Arachnoidea of interest medically the Acarina is far more 

 important than the Linguatulida. 



ACARINA. 



Of the acarines we are chiefly interested in the mites and the ticks. 

 The acarines do not show any separation of the abdomen from the 

 cephalo-thorax. A hexapod larva develops from the egg; this is suc- 

 ceeded by an octopod nymph which differs from the adult in not having 

 sexual organs. 



In addition to the four pairs of legs in the fully developed acarine there are two 

 other paired appendages, the chelicerae. in front of the mouth, and the pedipalps 

 on either side of the mouth. 



Trombidiidae. 



These generally have a soft, more or less hairy integument and are often brightly 

 colored. The two eyes are often pedunculated and the cheliceras are lancet shaped 

 and the palps project beyond the rostrum as claw-like appendages. A tip-like 

 appendage on the apical segment of the palps is characteristic. A very common 

 and annoying member of this family is the hexapod larva of the Trombidium 

 holosericeum. It is usually designated Leptus autumnalis. Popularly it is termed 

 "harvest mite," "red bug" or "jigger." They are found in the fields in the autumn 



