J. T. Wadsworth lol 



position which they retain as long as the pressure is applied. The 

 significance of this curvature will be apparent if we consider the structure 

 of the flower-head and the act of oviposition. 



As is well known, the young florets of composite flowers are protected 

 by an involucre which is composed of numerous bracts or scales. In 

 the flower-head of C. nigra these bracts arise just below the level of the 

 receptacle, and this is the region selected by the fly for the insertion 

 of its ovipositor. 



During oviposition the blunt end of the first segment of the ovi- 

 positor is placed between the lower scales as described above, and whilst 

 in this position the piercing portion is forced into the tissue of the 

 receptacle. The piercing segment passes downwards and inwards 

 towards the vertical axis of the flower- head, and gradually bends 

 upwards until the tip of the ovipositor finally reaches the space between 

 the tops of the young florets and the overlying bracts. The ova are 

 usually placed in this space ; less frequently, however, they are to be 

 found between the young florets. In the latter case the flower-heads 

 are probably older than in the former case, and the florets are longer ; 

 consequently the ovipositor is not sufficiently long to reach the space 

 wherein the eggs are usually laid. 



Goureau, who correctly described the external appearance and 

 movements of the ovipositor in U. cuspidata, believed that the ovipositor 

 was pushed between the florets of open flowers, and that the eggs were 

 then placed on the receptacle of the flower-heads. A similar opinion 

 was also held by Dufour who observed some phases of the life-history 

 of U. quadrifasciata. The latter observer, in addition, supposed that 

 the eggs were placed inside the tissue of the receptacle, and further, 

 he believed that the presence of the eggs, and possibly the injection 

 of an irritating fluid at the same time as these were laid, induced " une 

 irritation nutritive du receptacle transforme alors en placenta " ; in 

 other words induced the formation of the gall. 



Although the observations of these entomologists refer to species 

 other than the one considered here, yet, as these species are all so closely 

 related to each other, and their larvae also feed in similar flower-heads, 

 it is almost certain that on this point the opinions of these observers 

 were erroneous. 



From evidence obtained by opening the flower-heads after ovi- 

 position and counting the eggs, and also by counting the number of 

 larval chambers in galled heads, I believe the number of eggs usually 

 laid at one time varies from one to four ; where more than four eggs 



