36 U.S. NATIONAL MUSEUM BULLETIN 256 



consume a minimum quantity of food since they are still very small 

 in April. They become more active during that month and begin 

 to tunnel farther into the segment, but growth is not rapid. They 

 are about half grown in July, when development accelerates, and full 

 growth is reached in late August. The larvae of the other three species 

 do not possess this habit of remaining semiquiescent in a small cell, 

 but develop steadily, if rather slowly, during the winter months. 

 The cocoons of the unnamed form are usually found in the larval 

 feeding cavities, whether in destroyed or green segments. 



Unlike the other species, which are widely distributed, this insect 

 has a restricted range. It occurs from a line drawn from Uvalde to 

 San Antonio, Texas, south to the Rio Grande between Eagle Pass and 

 Rio Grande City, and for a distance of about 50 miles south of the 

 Rio Grande on the Mexican side of the border between the two last- 

 named towns. It has not been found in the coastal belt of southern 

 Texas east of long. 98° W., and its range to the west and northwest 

 does not extend into the foothills bordering the plateau of west Texas. 



The host plants are the closely allied bush prickly pears Opuntia 

 lindheimeri and 0. cacanapa. The former plant is very prevalent in the 

 Uvalde-San Antonio-Rio Grande sector, where it grows profusely in 

 large fields. Despite the abundance of its food plant, the insect is not 

 particularly common. Colonies of larvae are generally found in scattered 

 plants among hundreds of noninfested plants. Occasionally it has 

 been comparatively numerous in local areas near Laredo and at a few 

 other points. 



As in the case of M. dentata and M. doddalis, there is an annual 

 generation. Pupation takes place in late August and in September, 

 and moth emergence from late September to early November. The 

 females usually deposit an average of 60 to 75 eggs; for example: 125 

 females laid an average of 70 eggs 167-73 and 310 - 69. The average 

 number of eggs in a stick is 40 to 50; the highest count on record has 

 been 110 eggs in one stick. The incubation period may be as short as 

 10 days in early October, but for eggs deposited in late October and 

 early November the duration may be prolonged to as much as 44 

 days, the larvae hatching in the first half of December. 



In the more robust bush prickly pears such as Opuntia lindheimeri 

 and 0. inermis, the larvae of this insect, of M. doddalts, and of the 

 Texas form of M. prodenialis, tunnel downward from segment to seg- 

 ment, maintaining a clear communication channel from the lower 

 to the upper parts of the plant. They rarely leave their internal situ- 

 ation unless the food supply becomes exhausted. Cadoblastis cactorum, 

 on the other hand, prefers to vacate a segment to seek a new location 

 on the plant rather than to tunnel through from upper to lower seg- 



