208 MANUAL OP ZOOLOGY. 



except the anterior ones bearing two pairs ; by the composi- 

 tion of the antennae out of six or seven joints ; and by the 

 structure of the masticating organs, which consist of a pair of 

 mandibles without palps, covered by a lower lip, composed of 

 the confluent maxillae. 



In the common Millipede (lulus} the body is composed of 

 from forty to fifty segments, each of which bears two pairs of 

 minute, thread-like legs. The luli of this country are of small 

 size, but an American species attains a length of more than 

 half a foot. 



DISTRIBUTION OF MYRIAPODA IN TIME. About twenty species 

 of Myriapoda are known as fossils, the oldest example of the 

 order having been found in the Carboniferous Epoch. From 

 rocks of this age several species of Chilognathous Myriapods 

 have been discovered. They belong to the genera Xylobius 

 and Archiulus, and have been placed in a special family under 

 the name of Archiulidce. The occurrence of air-breathing 'arti- 

 culate animals (both Arachnida and Myriapoda) in the Carbo- 

 niferous period is noticeable, as being cotemporaneous with 

 the earliest known terrestrial Molluscs. 



CHAPTER XXXIX. 



INSECTA. 

 GENERAL CHARACTERS OF THE INSECTA. 



CLASS IV. INSECTA. The Insecta are defined as articulate 

 animals in which the head, thorax, and abdomen are distinct ; 

 there are three pairs of legs borne on the thorax; the abdomen 

 is destitute of legs ; a single pair of antennce is present ; mostly, 

 there, are two pairs of wings on the thorax. Respiration is 

 effected by tracheae. 



In the Insecta the body is divided into a variable number of 

 definite segments, or somites, some of which are furnished 

 with jointed appendages, and the nervous and circulatory 

 systems are constructed upon essentially the same plan as in 

 the Crustacea, Arachnida and Myriapoda. The head, thorax, 

 and abdomen are distinct (Jig. 61), and the total number of 

 somites in the body never exceeds twenty. ' Of these, five 

 certainly, and six probably, constitute the head, which pos- 

 sesses a pair of antennae, a pair of mandibles, and two pairs of 



