2S8 ARTHROPODS. 



Like other Arthropods, Insects have segmented . bodies, 

 jointed legs, chitinous armature, and a ventral chain of 

 ganglia linked to a dorsal brain. But what are their char- 

 acteristics ? Compared with Peripatus and the Myriopods, 

 adult Insects show concentration of the body-segments, 

 decrease in the number and increase in the quality of the 

 appendages, and wings withal. The young stages, however, 

 are often more primitive. 



Insects are terrestrial and aerial Arthropods, which are 

 usually winged as adults, which breathe air by means of 

 trachea, and often have a metamorphosis in the course of their 

 growth. The adult body consists ( i ) of a head with three pairs 

 of appendages (^legs ) and a pair of pre-oral outgrowths — the 

 antennae or feelers, (2) of a thorax of three segments, each 

 with legs, the last two with wings, and (3) of an abdomen 

 with segments, but with no legs unless rudimentary modifi- 

 dations of these be represented by stings, ovipositors, etc. 

 There are usually compound eyes and simple ocelli as well. 

 The genital ducts open at the end of the body. The average 

 habit is an intense activity. As this class is a very large one, 

 we shall revise its characteristics first in general terms, and 

 then as illustrated in the common cockroach. 



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 segments, as it bears three pairs of append- 

 ages. 



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



thoracic segments, each with a pair of legs, the two 

 last with wings. 



3. The abdomen with about eleven rings, usually with- 



out trace of limbs. 



But this is only the crude anatomy of form. You must 

 think of the long dragon-fly with outspread wings, and of the 

 compact cockchafer, of the thin-waisted wasps and long- 

 bodied butterflies, of house-fly and cricket, of large moths 

 and beetles, and the almost invisible insect parasites. 



Appendages.— \m&<A% "feel their way," test food, and 

 apparendy communicate impressions to one another by 

 means of a pair of jointed feelers or antennae, situated in 

 front of the head. Unlike the organs of a similar name in 



