THE SPIKE-HORNED LEAF-MINER. 11 



From the observations made in widely separated localities by both the 

 senior and junior authors, it appears that about 60 per cent of the 

 eggs deposited are fertile. 



EGGS DEPOSITED IN ONE LEAF AND ONE PLANT. 



In our breeding experiments at Columbia, S. C, 28 eggs have been 

 found in a single leaf of millet 6 inches tall, and as many as 50 eggs 

 in an 8-leaved plant; 39 in a 6-leaved plant; and 99 in a 9-leaved 

 plant. It can be seen very readily that even in case some of the 

 eggs are infertile or for other reasons do not hatch, the plants thus 

 heavily infested will be completely destroyed by the larvae. 



INCUBATION OF EGGS. 



At Tempe, Ariz., Mr. Wildermuth's records made during the 

 month of April showed the period of incubation to vary from 5 to 9 

 days, with an average of about 6 days. At Columbia, S. C, the senior 

 author's observations during July and August showed this period to 

 vary from 3 to 5 days, with an average of about 3 \ days ; one egg, 

 however, was found to hatch in 54 hours (2.2 days). In the fall this 

 period was found to vary from 4 to 8 days, with an average of about 

 5 days. At Glendale, Cal., the junior author made observations 

 during the month of March which extended this period from 7 to 

 12 days, with an average of 10J days. Observations continued dur- 

 ing both winter and summer months at Glendale and Pasadena, Cal., 

 showed the incubation period to vary from 5 to 12 days, with an 

 average of about 7 days. This period is without doubt somewhat 

 shorter in the open fields where infested plants are subjected to 

 refracted heat. 



LARVAL HABITS. 



The larva, when ready to emerge, ruptures the cephalic end of the 

 eggshell, as do the larvae of Argromyza angulata, and A. parvi- 

 comis, and immediately begins to feed on the green tissues of the leaf. 

 The mine at first is very small and threadlike, scarcely noticeable to 

 the unaided eye: (PL II, fig. 5.) The diameter of the mine increases 

 as the larva increases in size, and by the time, the larva reaohes 

 maturity the mine may be greatly widened. In large plants, with 

 long, wide leaves, the larvae frequently make mines from 15 to 20 

 inches in length. Such mines are usually linear in outline, and 

 although they run from side to side, the turns are less frequent than 

 when larvae mine in short leaves of smaller plants. In the latter 

 the larvse traverse the leaves oftener. They frequently make side 

 galleries diagonally across the leaves, then retreat and continue the 

 main mine down the blade. 



