394 



LECTURE XVIII. 



of a darker tint than the rest of the body (/>), and retains this cha- 

 racter, which is of use in determining the now numerous and rapidly 

 produced segments behind it. 



The antennae first begin to move, then the legs, and the first 

 instinct of the locomotive larva is to shun the light. In this pro- 

 gression the anal segment expands, and attaches itself to the lirni 

 surface ; then the body is carried forwards, the motion being pro- 

 pagated from segment to segment. On the twent^'-sixth day the 

 young lulus casts off the covering in wliich it had hitherto been 

 enfolded and enters the fourth period of development ; having now 

 seven pairs of legs and fifteen segments to its body {Jig. 156.). The 

 antenna? are elongated and exhibit six 

 joints; the eye is a single ocellus, but 

 is surrounded by pigment preparatory 

 to subdivision ; the new legs {b, c) are 

 as long as the old ones, but not so strong ; 

 the transverse markings of the primary 

 segments are more distinct ; the patch 

 on the seventh (p) is darker. The first 

 of the new segments (8) is almost equal 

 to the seventh primary one ; the penul- 

 timate (14) and anal (15) primary seg- 

 ments have not much enlarged. In the 

 progress of growth new segments are 

 successively added at the germinal space 

 (g), being always produced beneath the common integument, which 

 is afterwards moulted. These segments are added in a certain 

 numerical ratio, six at a time, between the antepenultimate and the 

 penultimate segments. ^Movements of the larva are always observed 

 to be fettered by the approach of the ecdysis. All the limbs super- 

 added to the primary three pairs are bifid; and these double legs 

 are homologous with the prolegs of caterpillars. The further course 

 of growth is attended with a more distinct definition of the segments, 

 and by transverse indents of the primary segments. The limbs also 

 become more straightened. The whole period of development oc- 

 cupies four or five weeks, and then development is superseded by the 

 mere act of growth. 



With regard to the Centipedes, we still need a series of researches 

 to make us properl}- acquainted with their development. In the month 

 of ^lay, the larva of Lithobius — a modified centipede, consisting of 

 seventeen segments, and having fifteen pairs of extremities — presents 

 but ten joints and seven pairs of legs with two simple ocelli on each 



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