8 ENTOMOLOGY FOR MEDICAL OFFICERS 



All the segments of an Arthropod are formed on one 

 plan, though that plan is subject to a good deal of modifica- 

 tion. The simplest form of segment is a ring, composed of 

 a dorsal arc, or notum (i/wtoi/ = back), and a ventral arc, or 

 sternum {sTTepvov = \>XQ&s\); between the notum and sternum 

 there may be two side pieces or pleura (TrXeuyoo'i/ = flank). 

 The pleura are sometimes quite inconspicuous, as in the 

 abdomen of an insect ; sometimes large and complex, as in 

 the thorax of many insects. 



Between the segments, when they are not modified by 

 fusion, the cuticle remains soft, to give freedom of move- 

 ment. 



The " typical " segment carries a pair of appendages 

 which are articulated near the junction of the sternum with 

 the pleuron on either side. 



In all Arthropoda, except a few degraded parasites in 

 which all traces of segmentation are lost, the anterior 

 segments are indistinguishably united to form a head, and 

 their appendages are modified, wholly or partly, for dealing 

 with food, and for sensory purposes. 



Behind the head the segments and appendages may be 

 all alike, or nearly so, as for instance in Centipedes ; or the 

 segments may, to a varying extent, be fused together, and 

 their appendages diversely modified and some of them 

 suppressed altogether, as for instance, in a crab, or a scorpion, 

 or an insect. 



The cuticle of Arthropoda consists largely of a tough 

 substance called chitin {j(iruiv = a coat of mail), and is 

 sometimes further strengthened by calcareous deposit. 

 Chitin is extremely resistant ; it withstands the action of 

 strong acids and alkalis, and is decomposed only by pro- 

 longed maceration. 



The outer layer of the cuticle is moulted periodically 

 en masse, either throughout the whole life of the animal, or 

 during the pre-adult stages of growth. For some little time 

 after each moult the cuticle is soft, but it gradually becomes 

 hard again. 



This chitinous cuticle is not a true skin like the 

 epidermis of vertebrate animals, since it is not composed of 

 cells but is merely a secretion of the epidermis. The same 



