CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 5 



The head bears a pair of ringed antennae (Fig. 6) ; behind these on 

 the sides of the head, there is a pair of short appendages termed oral 

 papillas. The mouth opening is surrounded by a row of lobes which 

 constitute the Hps, and between these in the anterior part of the 

 mouth-cavity there is an obtuse pro- 

 jection, which bears a row of chitinous 

 points. Within the mouth cavity there 

 are two pairs of hooked plates, which 

 have been termed the mandibles, the 

 two plates of each side being regarded 

 as a single mandible. 



Although the body is unsegmented 

 externally, internally there are evi- 

 dences of a metameric arrangement of 

 parts. The ventral nerve cords, which 

 at first sight appear to be without ^r/ 

 ganglia, are enlarged opposite each 

 pair of legs, and these enlargments 

 are regarded as rudimentary ganglia. Fig. e.-Ventral' view of the head 

 We can, therefore speak of each sec- and first pair of legs of Peri- 

 tion of a body corresponding to a g[SS.'' "" ^"''''"^' ''' °'^ 

 pair of appendages as a segment. The 



metameric condition is farther indicated by the fact that most of 

 these segments contain each a pair of nephridia; each nephridium 

 opening at the base of a leg. 



The respiratory organs are short tracheae, which are rareiy 

 branched, and in which the taenidia appear to be rudimentary.* In 

 some species, the spiracles are distributed irregularly ; in others, they 

 are in longitudinal rows. 



The sexes are distinct. The reproductive organs open near the 

 hind end of the body, either between the last or the next to the last 

 pair of legs. 



The various species are found in damp situations, under the bark 

 of rotten stumps, under stones or other objects on the ground. They 

 have been found in Africa, in Australia, in South America, and in the 

 West Indies. 



Their relationship to the Arthropoda is shown by the presence of 

 paired appendages, one, or perhaps two, pairs of which are modified as 

 jaws; the presence of tracheae which are found nowhere else except 



*It is quite possible that the "short tracheae" described by writers on the 

 structure of these animals are tracheoles. See the account of the distinguishing 

 features of tracheae and tracheoles in Chapter III. 



