26 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



CHARACTERS THAT DISTINGUISH INSECTS FROM OTHER ANIMALS. 



Insects are articulated animals having six legs. They com- 

 prise the class Hexapoda (Insecta) of the phylum or branch 

 Arthropoda, to which the spiders, centipedes, and Crustacea also 

 belong. Certain dififerences in structure may serve to distinguish 

 insects from their allies, and some of these are set forth in the 

 following table : 



Phylum Arthropoda. 



Animals with a tough or hardened exterior, transversely seg- 

 mented, and bilaterally symmetrical. 



1. Respiratit)n aerial, by tracheae or lungs 2 



Respiration aquatic, by gills or by the general integu- 

 ment ; two pairs of antennae ; head and thorax usually 

 consolidated Crustacea 



2. Head distinct 3 



Head and thorax united to form cephalothorax ; ab- 

 domen distinct; antennae wanting; eight legs. .Arachnida 



3. Head, thorax, and abdomen distinct ; six legs and one 



pair of antenna Hexapoda 



Thorax and abdomen not differentiated, formed of a 

 large number of similar segments, each bearing one 

 or two pairs of legs ; one pair of antennae .... Myriapoda 



Qass Hexapoda or Insecta includes all of the insects — a 

 class far more numerous in species than any other class of 

 animals. Except in some specialized forms, all have six legs in 

 the adult stage. 



Myriapoda. Centipedes and millipedes, commonly known as 

 thousand-legged worms. The large number of legs and the 

 lack of differentiation of thorax and abdomen serve to distinguish 

 this from other classes. 



Arachnida. Spiders, mites, scorpions, and ticks. Though 

 of some diversity of form, the lack of antennae and the number 

 of legs make it easy to recognize the members of this class. 



Crustacea. Crayfish, lobsters, crabs, etc. Though some 

 members of this class live out of water, they still breathe through 

 gills or through the integument and live in moist places. The 

 Crustacea have typically two pairs of antennae. The highest 



