GENERAL NOTES ON INSECTS. 301 



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 undivided head, which consists of at least three fused seg- 



ments, as it bears three pairs of appendages besides the antennee. 



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



segments, each with » ' pair of legs, the last two often with 

 wings. 



3. The abdomen with about eleven rings, usually without trace of 



limbs. 

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

 fly with its large outspread wings, the compact cockchafei, the thin- 

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

 the laige 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, which some authorities regard as 

 pre-oral outgrowths, not as true appendages. Then follow 

 the mandibles, first maxillae, and second maxillae, on the 

 head; the three pairs of legs on the thorax; and some- 

 times 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 were 

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



(1) Furthest 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 <£az 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 maxillae 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 maxillae, 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 



