518 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



earthen walls with a thin layer of silk and becomes transformed into 

 a pupa. 



At the time when the cell is closed and when, consequently, the 

 feeding of the larva terminates, the latter does not appear to have 

 entirely lost its desire for food. It devours eagerly all the animal 

 prey which is offered to it. This makes it necessary to conclude that 

 the mother wasp has a special instinct which leads her to suspend the 

 alimentary functions of the larva at the proper time. We shall see 

 presently, indeed, that under certain circumstances she herself delays 

 the closing of the cell and prolongs for a considerable time the rear- 

 ing of a single larva. The insect is informed by a very special in- 

 stinct of the time when she should wean her offspring. 



Under ordinary conditions about a month is required by S. cornuta 

 to rear her larva from the egg to the closing of the cell in which it is 

 contained. Thus, at Brazzaville my assistant, Mr. Weiss, determined 

 the presence of an egg in a newly made cell on October 29, but it 

 was only on November 2G that the female began to close the orifice 

 of this compartment, having finished the feeding of her larva. I do 

 not know the amount of time which it is necessary to deduct for the 

 development of the egg in order to arrive at the exact duration of 

 the larval life to the end of feeding. This period should, moreover, 

 vary a good deal according to the abundance of the nourishment 

 which the larva receives. Nests of four cells are frequently found 

 in which the first three cells are walled up. while the female has al- 

 ready begun the rearing of a fourth larva and no hatching of the 

 adult has yet taken place. Since as it is necessary to count 20 days 

 from the closing of the cell to the hatching of the adult there is rea- 

 son to believe that the larval growth was very rapid in such cases, as 

 the three larvse were reared before the end of the period. 



At the laboratory of the commission at Brazzaville, I followed 

 the history of a nest which was begun under my eye at the begin- 

 ning of July. This nest, which was built by a young female, was 

 limited to a single cell until October 20, when the female decided to 

 wall in the larva and begin the construction of the next cell. Dur- 

 ing more than three months, therefore, the wasp was occupied in 

 nourishing and caring for the same larva. I frequently saw it on the 

 nest, assuring itself of the solidity of the cell, and inspecting the sur- 

 roundings, evidently concerned by the necessity of walling up the 

 first cell and building another to receive the second egg. I found an 

 explanation a little later of the exceptional length of this particular 

 period of rearing a larva. 



On November 26 I opened the cell, which was closed at this time, 

 and found the place of the pupa occupied by a parasitic ichneumon- 

 fly. The slow growth of the infested Synagris was thus easily ex- 

 plained. From the facts mentioned, the conclusion must be drawn 



