10 THE CLASS OF INSECTS. 



num each consist, in the embryo, of two Lateral parts, or lialves, 

 which, during development, unite on the median line of the 

 body. Typically, therefore, the crustacean ring consists pri- 

 marily' of eight pieces. The same number is found in all insects 

 which are wingless, or in the larva and pupa state ; this applies 

 also to the Mja-iapods and Spiders. 



In the Mja-iapoda, or Centipedes, the broad tergum overlaps 

 the small epimera, while the sternum is much larger than in 

 the Spiders and Insects. In this respect it is like the broad 

 flat under-surface of most worms. Hence the legs of the 

 Centipede are inserted very far apart, and the "breast," or 

 sternum, is not much smaller than the dorsal part of the crust. 

 In the Jidus the dorsal piece (tergum) is greatly developed 

 over the sternum, but this is a departure from what is ap- 

 parently the more tj^^ical form of the order, i. e. the Centipede. 

 In the Spiders there is a still greater disproportion in size 

 between the tergum and the sternum, though the latter is verj'- 

 large compared with that of Insects. The epimera and episterna, 

 or side-pieces of the Spiders, are partiall}- concealed b}' the 

 over-arching tergum, and they are small, since the joints of the 

 legs are very large, Audouin's law of development in Ai'ticu- 

 lates showing that one part of the insect crust is alwaj's 

 developed at the expense of the adjoining part. In the Spider 

 we notice that the back of the thorax is a single solid plate 

 consisting originally of fom- rings consolidated into a single 

 hard piece. In like manner the broad solid sternal plate 

 results from the reunion of the same number of sternites cor- 

 responding, originally, to the number of thoracic legs. Thus 

 the whole upper side of the head and thorax of the Spider is 

 consolidated into a single hard horny immovable plate, like 

 the upper solid part of the cephalothorax of the Crab or 

 Shrimp. Hence the motions of the Spiders are very stitf com- 

 pared with those of many Insects, and correspond to those of 

 the Crab. 



The crust of the winged insect is modified for the per- 

 formance of more complex motions. It is subdivided in so 

 different a manner from the two lower orders of the class, that 

 it would almost seem to have nothing in common, structurally 

 speaking, with the groups below them. It is only by examin- 



