spotted fever of sheep and cattle, to which human beings are, how- 

 ever, susceptible. The virus is a micro-organism Dermacentroxenus 

 rickettsi and is transmitted by the tick Dermacentor venustus. 



Insecta or Hexapoda constitute one of the largest groups not 

 only of arthropods but of the whole animal kingdom. The plan 

 of segmentation, which may be exemplified by any common insect, 

 such as the cricket, Gryllus, exhibits a division of the body into 

 three regions, the head, thorax and abdomen. The head bears 

 a pair of antennae and the large compound eyes, together with 

 three pairs of mouth parts, modified in the case of Gryllus for 

 cutting plant material. The thorax bears three pairs of walking 

 legs, associated with three divisions of the thorax itself, respec- 

 tively the prothorax, mesothorax and metathorax, and there 

 are typically two pairs of wings, the occurrence and form of which 

 are subject to profound modifications. The abdomen is segmented 

 consisting of some ten segments, lacking appendages except certain 

 special ones serving the function of reproduction. Respiration is 

 by means of air tubes or tracheae, but there are various modifica- 

 tions in forms which are aquatic. 



An extensive metamorphosis is one of the outstanding features 

 of the group, especially significant in its modifications in parasitic 

 forms. Typically, as in the cabbage butterfly, Pieris, the female of 

 the adult insect, here the air-living winged form, produces eggs 

 which on hatching give rise to a larva "caterpillar" phase, the form 

 of the body of which resembles rather closely that of the annulate 

 worm. It is an actively feeding phase, a very destructive one in 

 the case of many allied forms living on cultivated plants, which in 

 time is replaced by a quiescent phase, known as a pupa, and often 

 protected by an enclosing case or cocoon spun by the larva. From 

 the pupa there emerges finally the adult or winged sexual form. 



The life history with its seasonal, sex, and food modifications is 

 exceedingly various in the insect group, and its details are of 

 significance in the case of a very large number of species which are 

 either parasitic or injurious otherwise. Of the parasites, some, 

 such as various species of mosquitoes (Culex), blackflies {Simulium), 

 and bed-bugs are temporary but irritating parasites of man. 

 Others, such as the lice (Pediculus, Phlhirius) and fleas (Pulex), 

 are more or less permanent parasites of the skin, while many, such 

 as the malarial mosquitoes {Anopheles) and the tsetse flies (Glos- 

 sina) are significant both as surface parasites and as the carriers of 

 disease, the primary agents of which are the lower organisms. 

 Some species, such as the bot-flies, and screw-worm flies, have 

 parasitic larval phases. In the bot-fly of the horse, Gastrophilus 

 equi, the adult female deposits the eggs upon the hairs of the host 



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