4 Prof. J. C. Schiodte on the Structure of 



These prolongations take rise from the sides only of the 

 rings of the body — that is, from the pleurae or lateral folds be- 

 tween the dorsal and ventral plates ; and in that case the 

 pleurae may be covered to a greater or less extent by a de- 

 posit of chitine forming a lateral plate, called the epimeron, 

 which affords support for the muscles which move the basal 

 joint of the limb, the hip or coxa. 



It is also from the pleurae alone that the peculiar prolonga- 

 tions arise which are in the service of respiration, either out- 

 side the body as gills or branchiae, or inside as tracheae. 

 From the dorsal and ventral arches of the rings, on the con- 

 trary, no limbs ever take rise, although they often carry other 

 more superficial formations (which may also be subservient to 

 locomotion) for the pui-pose of supporting or fixing the body, 

 such as groups of sharp granules, indentations, spines, retrac- 

 tile warts, often with hooks disposed in circles, &c. To this last 

 class belong the so-called ventral or false feet of many larvae. 

 The cerci of Insects with perfect metamorphosis (both of imagos 

 and of larvae) are merely transformed dorsal spines. 



The head consists, as all tlie other rings of the body, of a 

 dorsal arch, a ventral arch, and two side pieces. In this case, 

 too, it is merely from the side pieces that the limbs take rise, 

 namely the appendages of the mouth, and, besides, the eyes and 

 antennae. But as the head, besides one or two pairs of an- 

 tennae, possesses three pairs of limbs (appendages of the 

 mouth), whilst none of the other rings have more than one 

 pair, the lateral pieces or pleurae of the head (cheeks and 

 temples) preponderate so greatly over the dorsal and ventral 

 pieces, that they occupy the greater part of the skull, particu- 

 larly behind and above, where so much space is required for 

 the accommodation of the muscular and nervous systems ; 

 whilst the dorsal and ventral pieces lose so much the more in 

 extent as they are here relieved from the office they perform in 

 all the other rings (saving the last of all), viz. to afford sur- 

 face of insertion for the muscles which move the next fol- 

 lowing ring. The dorsal piece is therefore reduced to a plate 

 above the mouth (the epistoma), from which separate pieces, 

 viz. clypeus, labrum, labellum, are evolved in proportion as the 

 mouth requires covering from above. The ventral piece, on 

 the contrary (hypostoma, or the sphenoid plate, as it may be 

 called from its analogy to the os sphenoideum of vertebrata), 

 enters into closer relations with the appendages of the mouth, as 

 will be shown more in detail by-and-by ; from this, too, sepa- 

 rate pieces are evolved in front, namely nientum and fulcrum 

 labii, in proportion as the maxillae and lingua with their muscles 

 require support and cover from beneath. 



