ITS ZOOLOGICAL POSITION. 13 



Characters of Insects. 



Insects are distinguished from other Arthropoda by the 

 arrangement of the segments of the body into three plainly 

 marked regions head, thorax, and abdomen ; by the three 

 pairs of ambulatory legs carried upon the thorax ; by the single 

 pair of antennae ; and by the tracheal respiration. Myriopods 

 and Arachnida have no distinct thorax. Most Crustacea have 

 two pairs of antennae, while in Arachnida antennoe are wanting 

 altogether. Crustacea, if they possess special respiratory organs 

 at all, have branchiae (gills) in place of tracheae (air-tubes). In 

 Arachnida, Myriopoda, and Crustacea there are usually more 

 than three pairs of ambulatorv leo:s in the adult. 



JL * 



The appendages of an Insect's head (antennae, mandibles, 

 maxillae) are appropriated to special senses, or to the operations 

 of feeding, and have lost that obvious correspondence with 

 walking legs which they still retain in some lower Arthropoda 

 (Peripatus, Limithis, Arachnida). The thorax consists of three* 

 segments, each of which carries a pair of ambulatory legs. No 

 abdominal legs are found in any . adult insect. The middle 

 thoracic segment may carry a pair of wings or wing-covers, and 

 the third segment a pair of wings. 



The lower or less-specialised Insects, such as the Cockroach, 

 have nearly as many nerve-ganglia as segments, and the longi- 

 tudinal connectives of the nerve-cord are double. In the adult 

 of certain higher Insectsf (e.g., many Coleoptera, and some 

 Diptera) the nerve-ganglia are concentrated, reduced in number, 

 and restricted to the head and thorax ; while all the con- 

 nectives, except those of the oesophageal ring, may be outwardly 

 single. 



The heart, or dorsal vessel, is subdivided by constrictions 

 into a series of chambers, from \vhich an aorta passes forwards 

 to the head. 



Air is usually taken into the body by stigmata or breathing- 

 pores,^: which lie along the sides of the thorax and abdomen. 



* In some Insects there are traces of a fourth thoracic segment, 

 f So also in some larvze (Calandra, (Estrus, &c.). 



In some aquatic Insects the exchange of gases is effected by "pseudobranchire," 

 and the tracheal system is closed. 



