182 Dr. T. A. Chapman on Egg-laying of Trichiosoma. 



in separating the layer of cuticle, what has to be cut through 

 is the vertical walls of the cells of the leaf, which being only 

 the divisions between the cells, have much less area than 

 the whole of the pocket, and being very thin and (in these 

 young leaves) very soft and tender, are easily caught 

 between the projecting blades of the saws and cut through. 

 The actual beginning of the process, the entrance of the 

 terebra into the leaf, was always obscured by the portion 

 of the sheath referred to already. It always took a time 

 greater in proportion, one felt, than the cutting of the pocket 

 afterwards. This resulted, no doubt, from the fact that 

 cutting had here to take place over the whole line of advance 

 and not merely at the widely separated dissepiments of 

 the leaf cells. The flies would only use young and tender 

 (in fact, not fully grown) leaves, and whilst this would 

 facilitate the cutting at all stages of the process, it would be 

 important at the first penetration, as the saws could not 

 cut unless the cuticular tissue was soft enough for them in 

 some degree to indent it. This advantage or necessity 

 would of course be much the same whatever the precise 

 manner of cutting was. The analogy or rather identity 

 with scissor action, or multiple scissor action as in horse- 

 clippers, is perhaps more easily realised when it is called to 

 mind that the two saws are strengthened lattice-girder 

 fashion by transverse thickenings on their outer surfaces, 

 which are thus uneven and irregular, but that their opposed 

 faces are quite flat, sliding smoothly on each other precisely 

 as is the case with the opposed faces of scissors or clippers. 

 Mr. Morice has added to my indebtedness to him in the 

 matter of this Trichiosoma and its correct position as a 

 species, by giving me the photographs on a much enlarged 

 scale of the extremity of the terebra, and still more enlarged 

 of its cutting edge, which are reproduced on PI. XII. 



Explanation of Plates X— XVI. 



Plate X shows Trichiosoma in its relative position to the leaf 

 when ovipositing. Photograph from a specimen mounted on a leaf 

 that had an egg laid in it. The pocket from which the larva duly- 

 hatched is seen on left, opposite end of wing. The terebra is in 



