December, '14] 



MCGREGOR: COTTON LEAF-MINER 



449 



The life history on cotton has not been determined in all details, 

 but in a general way it is fairly well established as Avill be shown by 

 the following notes: 



Egg 



The actual deposition of the egg has not been observed on cotton. 

 On several occasions numerous adults of both sexes were liberated 

 under control upon cotton leaves. On these occasions many puncture 

 marks were made on the upper side of the leaves, but no oviposition 

 was effected. These incisions resemble precisely the ovipositional 

 punctures which are always to be found at the point of origin of 

 the tunnel, and it is very probable that they are made to incite the 

 exudation of sap for feeding purposes. Thus, unlike oviposition in 

 alfalfa,^ the leaf-miner invariably inserts her eggs on the upper surface 

 of the cotton leaf. 



In the case of cotton, the leaf-miner adult makes a very character- 

 istic egg-puncture in the leaf (see fig. 27, a, b, and c). A hole is first 

 rasped through the dorsal epidermis 

 and then a small chamber is excavated 

 in the underlying .palisade tissue. The 

 formation of this chamber causes the 

 overlying epidermis to become counter- 

 sunk, thus giving somewhat the ap- 

 pearance of a crater or drum-head (best 

 shown in fig. 27, c). It is, doubtless, 

 into the innermost recess of this cham- 

 ber that the egg is inserted, since it is 

 always from this point that the tunnel 

 begins. Measurements of a consider- 

 able series of punctures yielded these 

 averages: longest axis, .138 mm.; short- 

 est axis, .0942 mm. 



Larva 



Upon hatching, the larva feeds almost exclusively upon the pali- 

 sade tissue near the upper surface, and consequently it is there 

 (plate 14, fig. 1) that the mine is formed. If the infested leaf is held 

 between the eye and the sun the entire feeding operations can be seen. 

 Holding to the tunnel walls by means of two curved hook appendages 

 (see plate 14, fig. 3, for the remains of these organs in the pupa) the leaf 

 tissue is rasped by the ceaseless activity of a remarkable black, radula- 



-1 ; .' -;?:,~^"»«l«^j'i.iv '■•'= 



Fig. 27 — a, b and c 



three ovipo^it'onal punctures by 



A gromyza scutellata, X 90 



1 Webster and Parks. The Serpentine Leaf-miner. Jom-. Agi-. Res., vol. I, no. 1, 

 Oct. 10, 1913. 



