156 Prof. J. Wood- Mason's Morphological Notes 



Scolopendrella representing the legs of insects, which 

 would appear to be endopodites.* 



The appendages of the legs, both in Machilis and 

 Scolopendrella, appear to be quite immovable, and I feel 

 sure that they are nothing but functionless rudiments. 

 Those of the abdomen, in 3Iachilis, on the contrary, are 

 movably attached ; the last and longest pair of them is 

 invariably used in ordinary locomotion, and it is by their 

 aid, at all events, that the sudden and powerful forward 

 leaps t which the creature executes on being touched are 

 effected ; the rest, though they frequently move forwards 

 and backwards Avith the hindermost pair, only succeed in 

 planting themselves upon such projections of the surface 

 over which the insect is passing as happen at the com- 

 mencement of the strokes to come within reach of their 

 shorter length. In their position of rest in the living 

 insect they all slope downwards and backwards ; but when 

 a stroke is to be made they must be brought from this 

 position to one in which they slope in the opposite direc- 

 tion, that is to say, forwards. 



These appendages are commonly said to be attached to 

 the posterior margin of the sternum, but a comparison of 

 the sternum and appendages of the second abdominal 

 somite with those of the metathorax shows that this is not 

 the case. In Machilis, or, better, on account of its larger 

 size, in such a cockroach as Panesthia javanica, the meta- 

 thoracic sternum is made up of a short and soft anterior 

 portion covered by the preceding somite, and of a hard 

 and triangular posterior portion, to each side of which are 

 articulated the two short basal joints that carry the five- 

 jointed limb. Let us suppose the two limbs to be alto- 

 gether aborted, rudimentary exopodites to be added in 

 their proper place, and the two basal joints to be indis- 

 tinguishably fused together, but to remain limited off from 

 the sternum by a distinct suture and divided from their 



* It should not be forgotten that, in the Mysis-stage of some prawns, 

 it is the endopodite, and not the exopoditc, of the thoracic members which 

 is small and simple : — " Die funf nenen Fusspaare sind zweiastig, der 

 innere Ast hurz, einfaeh, — dfir aUasere Idnger, wm etide gerlngelt" — 

 Fritz Muller, ' Fiir Dai'win,' S. 41, fig. 31. It" such a long exopodite were 

 to grow up into a five-jointed ambulatory limb, the simple endopodite 

 persisting, we should have precisely the condition of things we see in 

 Scolopendrella. 



t Supposing that all animals which now fly are descended from ances- 

 tors that jumped, it is interesting to find this wonderful saltatorial al)ility 

 and the beginnings of wings in the Crustacean-like pleural prolongations 

 of the thoracic terga of these ' Urinsekten.' 



