THE EGG-BAYING fil" PHYMATOCERA ATERRIMA, KLUG. 



147 



couie entirely from the narrow piece of terebra opposite to it, which is 

 much too small to hold it. Some trace of its movements may be seen 

 at the base of the terebra, but the egg certainly passes clown the 

 terebra to the place where it appears, as a very much narrowed and 

 therefore lengthened body, so that no trace of its passage in the terebra, 

 beyond the heaving movements of the parts, is visible ; so that when 

 it does appear, gradually protruding from the cutting luargm of the 

 tei-ebra, it appears to come from nowhere or to be suddenly materi- 

 alised from nothing. 



All this agrees with Mr. Morice's observations except as to two 

 items. One is that I never saw the fly withdraw the terebra from the 

 incision during the whole time she was occupied with it, but always 

 carried it back from finishing the portion being cut to the egg-laying 

 position with the tip within the incision, except on one occasion, when 

 1 happened to shake the stem so as to cause the terebra to leave the 

 slit. It was with a little delay and some awkwardness that the fly 

 managed to re-introduce it properly. In discussing this point with 

 Mr. Morice he assures me that in all his observations the terebra was- 

 removed and re-inserted as he describes. 



Mr. Morice describes the terebra when first introduced as being 

 Avorked straight in to its full depth at the position it will occupy when 

 the first egg is laid, that is to the position shown in fig. 2^. 



There are two difticalties in the way of supposing that this occurs. 

 The first is that all the incisions with eggs of Phi/uiatocera I have 

 seen show a step at the beginning of the slit, as shown diagrammati- 

 cally in figs. 2, 3, and 4. The other difficulty is that the terebra 

 seems to have no cutting power except in its distal half, by which it 

 can cut all the sections shown in my diagrams, but could not cut the 

 portions shown in figs. 1 and 2 at one operation, as it would have to 

 do if it made the complete penetration at first, nor does it seem probable 

 it would make this deep incision once only during the long process of 

 laying a series of eggs. 



The eggs when laid do not reach quite the bottom of the pocket, 

 but rest a little way above it, as shown in diagrams 4 and 5. 



It is also the case that at the line of incision the flap of cuticle 

 does not quite return to its original place, but leaves a slight gap, as 

 indicated in diagram 5. This seems a necessary result of the space 

 taken up by the egg, but may also be due, to some slight extent, to 

 contraction from d^iccation. 



To return to the actual process of cutting, we may take diagram 2, 

 in which the part above the dotted line is already cut, but the part 

 l)elow has to be cut, the terebra taking up the several positions 4, 5, 

 6, and 7 in doing so. 



Two photographs of the terebra are shown in Plate l\. In my 

 hands, the two plates of the terebia never remain, when mounted, in 

 their natural positions, one accurately applied to the other, so that 

 there would, in such photographs as those shown, seem to be only 

 one ))late, apart from any differences in the advancement of the sav,-. 



In my preparations (and photographs) the two plates are always 

 more or less dislocated ; this is not true to nature, but demonstrates 

 that there are two platet-. A photograph of the terebra undislocated 

 might suggest that there was only one plate. 



In the photographs the supports are to the left, and are not very 



