WHAT IS AN INSECT? S 



other words, the nervous chain forms an oesophageal ring, 

 which is threaded through by the gullet. 



The Arthropods are divided into four classes, which may 

 be distinguished as follows : — 



1. Crustacea. (Examples: — lobster, wood-louse, barnacle.) 



Nearly always live in water and breathe by gills. 

 There are two pairs of feelers. 



2. Arachnida. (Examples: — spider, scorpion, mite.) 



Nearly always air-breathers. The body is divided 

 into two regions, on the foremost of which all 

 the legs (not fewer than four pairs) are carried. 

 There are no feelers. 



3. Myriopoda. (Examples : — centipedes, millipedes.) 



Air-breathers, with tracheal tubes. There are 

 many segments behind the head, and these are 

 not grouped into regions. Each segment bears one 

 or two pairs of legs. There is one pair of feelers. 



4. Insecta. Air-breathers, with tracheal tubes. The 



segments are grouped into three regions, head, 

 thorax, and abdomen. The thorax bears three pairs 

 of legs. There is one pair of feelers. The 

 Insecta include the only winged Arthropods, but 

 some insects never acquire wings at all. 

 We must shortly explain what is meant by tracheal tubes in 

 the foregoing table. In some Arachnida, in Myriopoda, and in 

 Insecta, the air taken into the body for respiration is distributed 

 by a series of branched air-tubes, called tracheae, from their 

 supposed resemblance to the trachea or wind-pipe of a lung- 

 breathing vertebrate. These air-tubes are stiffened internally 

 by a spiral chitinous thread, which, though perfectly flexible, 

 prevents the tube from collapsing. India-rubber gas-pipes are 

 often lined with a spiral wire to prevent collapse when the tube 

 is accidentally trodden on, or bent at a sharp angle, and such a 

 spiral wire is not unlike the thread of an insect's trachea. 



Our answer to the question "What is an Insect?" may be 

 given in these terms. An Insect is an Arthropod which 

 breathes air by tracheal tubes, has one pair of feelers, a 

 body divided into three distinct regions, head, thorax, and 

 abdomen, and a pair of jointed legs on each of the three 

 segments of the thorax,* but on no others. 



* Exceptions occur in insect-Iarvse. 



