GENERAL CHARACTERS OF INSECTS. 307 



through the heart, and the finer arteries traced into the large claws 

 and legs. In the crab, the entire upper side of the carapace may be 

 removed by the point of a knife. The smaller Crustacea, especially 

 the water-fleas, may be examined alive under the microscope as trans- 

 parent objects. In the larger forms the stomach may be laid open by 

 the scissors in order to study its complicated structure. The eyes of 

 the lobster should be hardened in alcohol and fine sections made for 

 the microscope. This is an operation requiring much care and expe- 

 rience. Experts in embryology have sliced the eggs of certain Crusta- 

 cea and studied their embryology with great success. 



THE AIR-BREATHING ARTHROPODA (Centipedes, Spiders, 



Insects). 



General Characters of Insects. While in the worms 

 there is no grouping of the segments into regions, we have 

 seen that in most Crustacea there are two assemblages of 

 segments *' * a head-thorax and abdomen. In the insects 



o * 



there is a step higher in the scale of life, a head is separated 

 from the rest of the body, which is divided into three 

 regions, the head, thorax, and hind-body (abdomen). More- 

 over, the insects differ from the Crustacea in breathing by 

 internal air-tubes which open through breathing-holes 

 (spiracles) in the sides of the body. The six-footed insects 

 also have wings, and their presence is correlated with a 

 differentiation or subdivision of the two hinder segments 

 of the thorax into numerous pieces. 



The number of body-segments in winged insects is seven- 

 teen or eighteen i. e., four in the head, three in the thorax, 

 and ten or eleven in the hind-body. In spiders and mites 

 there are usually but two segments in the head, four in the 

 thorax, and a varying number (not more than twelve) in 

 the abdomen ; in Myriopods the number of segments varies 

 greatly {. e., from ten to two hundred. The appendages 

 of the body are jointed, and perform four different func- 

 tions i. e., the antennae are sensorial organs, the jaws and 

 maxillae are for seizing and chewing or sucking food ; the 



