12 ENTOMOLOGY FOR MEDICAL OFFICERS 



The Insecta are for the most part terrestrial and aerial 

 Arthropods, and they breathe by definitely-arranged tracheae. 

 The segments of the body, in the adult, are disposed in three 

 very well-defined regions — head, thorax, and abdomen. The 

 first pair of appendages are antennae, and the thorax carries 

 three pairs of legs, and usually, in the adult, two pairs of 

 wings. 



The Arachnida are, for the most part, terrestrial Arthro- 

 pods, and the majority of them breathe air, sometimes by 

 definitely-arranged tracheae, sometimes by other means. The 

 grouping of the segments of the body is not constant, but 

 commonly two regions — cephalothorax and abdomen — can be 

 recognised. The appendages never exceed six pairs, of which 

 four pairs are legs. There are no antennae. 



In classical text-books the Arthropoda are treated with 

 some regard to phyletic precedence, either in the order given 

 above, or — as seems neater — with the A rackm'da immedia.tely 

 following the Crustacea. 



It is generally allowed that the Crustacea should take the 

 lowest room. For although some of the highest develop- 

 ments of the Class (such as the Land-crabs) are, when viewed 

 with our ethically-tinged spectacles, as nobly organised and 

 as " intelligent " as any Arthropod except an Ant, yet the 

 morphological level of the class as a whole lies lower than 

 that of the typical insect. At any rate they stand farthest 

 back in the distant geological retrospect : numerous fossil 

 Crustacea have been found, among the very oldest remains 

 of undoubted animal nature, in the Cambrian rocks ; and 

 Crustacean remains are quite abundant in the later Silurian 

 system, where Arachnida (Scorpions and Eurypterus), Myria- 

 poda, and Insecta (Cockroaches) first make an appearance. 



The Arachnida stand clear of the others, but seem to be 

 closer to the Crustacea than to any of the other classes. 



The Myriapoda and Insecta and possibly Peripatus stand 

 together, and apart from the other two classes. Some 

 zoologists even include the Centipedes in one class with the 

 Insects. 



In this book, however, we propose to forget phylogeny, 

 and to deal with Arthropoda in quite an abnormal way, and 

 according to a sort of pathological warrant of precedence, in 



