36 



TEXT-BOOK OF ENTOMOLOGY 



FIG. 20. A pro- 

 thoracic leg of ( 'hirono- 

 mus larva; and pupa. 



FIG. 21. A, larva of Ephydra 

 californica : a. 1>, <, pupa. 



armature, the result of. the act of moving or pulling the body over 

 the uneven surface of the ground. 



There is good reason to suppose that such limbs arose from dyna- 

 mical causes, similar to those exciting the formation of secondary 

 adaptations such as are to be seen in the prop or supporting legs of 

 certain dipterous larvae, as the single pair of Chironomus (Fig. L'O) 



and Simulium, or the series of unjointed soft 



tubercles of Ephydra (Fig. 21), etc., which are 



armed with hooks and 



claws, and are thus 



adapted for dragging 



the insect through or 



over vegetation or 



along the ground. 

 Now by frequent 



continuous use of such 



unjointed structures, 



the cuticle would tend 

 to become hard, owing to the deposit of a greater amount of chitin 

 between the folds of the skin, until finally the body being elongated 

 and homonomously segmented, the movements of walking or running 

 would be regular and even, and we would have homonomously 

 jointed legs like those of the trilobites, or of the most generalized 

 Crustacea and of Myriopoda. 



In the most primitive arthropods, and such we take it were on 

 the whole the trilobites, rather than the Crustacea, the limbs were 

 of nearly the same shape, being long and slender and evenly jointed 

 from and including the antennas, to the last pair of limbs of the 

 abdominal region. In these forms there appear to be, so far as we 

 now know, no differentiation into mandibles, maxillse, maxillipedes, 

 and thoracic legs, or into gonopoda. The same lack of diversity of 

 structure and function of the head-appendages has survived, with 

 little change, in Limulus. In the trilobites (Fig. 1) none of the 

 limbs have yet been found to end in claws or forceps ; being in this 

 respect nearly as primitive as in the worms. Secondary adaptations 

 have arisen in Limulus, the cephalic appendages being forcipated, 

 adapted as supports to the body and for pushing it onward through 

 the sand or mud, while the abdominal legs are broad and flat, 

 adapted for swimming and bearing the broad gill-leaves. 



It is thus quite evident that we have three stages in the evolution 

 of the arthropodan limb ; i.e. 1, the syllid stage, of simple, jointed, 

 soft, yielding appendages not used as true supports (Fig. 19) ; 2, the 



