THE CLASS AND ORDERS OF INSECTS 175 



pair of feelers and in breathing by means of air-tubes, but 

 which carry paired limbs on nearly the whole series of their 

 trunk-segments, the foremost of these modified into sharp- 

 fanged " poison-feet " for killing prey. There are the DIPLO- 

 PODA (millipedes) sometimes confused with the last named, 

 like them elongate and many-legged, but readily distinguished 

 by the cylindrical form of their bodies, most of the segments 

 whereof are united in couples, so that the usually numerous 

 legs, carried right underneath the body, seem to be arranged 

 two pairs on one segment. Then there are the ARACHNID A 

 (scorpions, spiders, mites, etc.), in which the foremost pair 

 of developed appendages are never feelers, but seizing or 

 piercing organs used for feeding, while there are usually not 

 more than four pairs of legs. It is worthy of note that in 

 Crustacea, Arachnida and Millipedes, the reproductive openings 

 are situated far forward in the body, while in the Centipedes, 

 as in the Insects, they are close to the tail-end. 



In this brief summary of the leading arthropodan classes, 

 stress has been laid on those characters whereby they are 

 distinguished from insects ; the characters that may be 

 regarded as defining the latter class may now be definitely 

 set forth. 



CLASS INSECTA 



The Insecta (or Hexapoda) are Arthropoda in which the seg- 

 ments are grouped into three regions : the head, the thorax, 

 and the abdomen. The head carries usually a pair of com- 

 pound eyes, a pair of feelers, and three pairs of jaws : mandibles 

 maxillae, and labium ; a pair of small maxillulae may also be 

 recognizable behind the mandibles, associated with the median 

 tongue. The thorax consists of three segments, each of which 

 bears a pair of legs and the two hinder usually carry also each 

 a pair of wings. The abdomen consists of eleven or fewer 

 segments, from most of which limbs are usually absent ; when 

 present these (except in some few cases the hindmost, cerci) 

 are always (in adult insects) much shorter than the thoracic 

 legs. Limbs (cerci and stylets) usually short, are not uncom- 

 monly present at the tail end ; processes of the eighth and 

 ninth abdominal segments usually serve as accessories to the 



