APPENDAGES 385 



General Notes on Insects 



The main characteristics of insects have already been 

 described in the two types chosen, but we here revise them 

 in general terms. 



Form. — The body of an adult insect may be divided into 

 three distinct regions : — 



1. The head, probably consisting of seven fused segments. 



2. The median thorax, divided into pro-, meso-, and meta-thoracic 



segments, each with a pair of legs, the last two often with 



wmgs. 



3. The abdomen, usually with ten to eleven segments, with never 

 more than transformed traces of appendages. 

 Within these limits there is great variety of form, e.g. the long 

 dragon-fly with its large outspread wings, the compact cockchafer, the 

 thin-waisted wasps and long-bodied butterflies, the house-fly and 

 cricket, the large moths and beetles, and the almost invisible insect 

 parasites. 



Appendages. — Insects feel their way, test food, and 

 apparently communicate impressions to one another, by 

 means of the antennae. Then follow the mandibles, first 

 maxillae, and second maxillae, on the head ; the three pairs 

 of legs on the thorax ; and sometimes vestiges of legs on 

 the abdomen. 



It was a step of some importance in morphology when Savigny 

 showed that the three pairs of appendages about the mouth are 

 homologous with the other appendages, i.e. are masticatory legs. 



(i) Farthest forward lie two mandibles, the biting and cutting jaws. 

 These are single-jointed, and thus differ from the organs of the same 

 name in the crayfish, which bear a three-jointed palp in addition to the 

 hard basal part. In those insects which suck and do not bite, e.g. 

 adult butterflies, the mandibles are reduced. 



(2) Next in order is the first pair of maxilla. Each maxilla consists 

 of a basal piece (protopodite), an inner fork (endopodite), and an outer 

 fork (exopodite). The entomologists divide the protopodite into a 

 lower joint, the cardo, and an upper, the stipes ; the endopodite into an 

 internal lacinia and an external galea ; while the exopodite is called 

 the maxillary palp. 



(3) The last pair of oral appendages or second maxillcB are partially 

 fused, and form what is called the labium. The lower and upper 

 joints of their fused protopodites are called submentum and mentum ; 

 the endopodites on each side are double, as in the first maxillas, and 

 consist of internal lacinia and external paraglossa ; the exopodites are 

 called the labial palps. 



The three pairs of thoracic legs consist of many joints, are usually 

 clawed and hairy at their tips, and differ greatly according to their uses, 



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