CACTUS-FEEDING INSECTS AND MITES 77 



formation on fecundity, since under cage conditions the females have 

 usually failed to oviposit. In one instant 1 158 eggs were obtained from 

 15 females, giving an average of 77.2 eggs per individual. 



The eggs, flat and either circular or oval in outline, are deposited on 

 the surface of the segment or along the spines. Eggs deposited on the 

 fiat surface of the plant are circular and are placed in groups of 40 to 

 90 with the edges overlapping. On the spines they are usually end to 

 end in a row, pressed tightly against each other, and assuming an oval 

 shape. The incubation period has been as short as 4 to 5 days at Uvalde 

 in June 1 926, while at San Luis Potosi in June 1 930 the period was 

 11 to 13 days. 



Immediately after hatching, the larvae spin a heavy protective web, 

 usually near an areole. They may live beneath this web for several days, 

 during which time they gnaw holes in the cuticle. All larvae from one 

 batch of eggs enter the plant through one hole, and younger colonies 

 may contain 50 or more individuals. As development proceeds, the 

 colony either divides or else suffers considerable mortality, since older 

 larvae are found in groups of a few to about 30, the average number 

 being about 10 larvae in a colony. They work downward, tunneling 

 through several segments and the main stem to near the base of the 

 plant. In semitree prickly pears with a definite trunk they inhabit a 

 cavity in the form of a long cylindrical tube weakly lined with silk in the 

 center of the main stem. The cavity is kept free from excreta; hence the 

 work in the main stems is not accompanied by rotting. One discharge 

 hole serves the colony for the ejection of frass pellets, and this hole may 

 be situated up to several feet above the point where the larvae are 

 feeding. The larvae are disinclined to transfer from one plant to another, 

 or even to move to another part of the same plant. In cages, if the in- 

 fested plant decayed, they usually died without making any attempt to 

 enter an adjacent plant. 



The flimsy cocoons are spun, generally in small clusters, within 

 the larval tunnels. The pupal stage has been determined to occupy 

 about 30 days in June. In tlie case where the summer generation 

 completed the life cycle in 52 days, the pupal period was approxi- 

 mately 14 days in late July and early August. Megastes larvae are cap- 

 able of causing considerable destruction to shrub pears, such as 0. 

 discala and 0. cantabrigiensis, and to the more succulent semitree pears, 

 such as 0. robusta. In typical semitree pears like 0. streptacantha, the 

 tunneling of die larvae effects no greater damage than the weakening 

 of woody main branches and stems, which may be broken off by wind. 

 Parasites. — The large larvae are often heavily parasitised by the 

 braconid Apanteles megathymi and the tachinids Phorocera sp. and 

 Lespesia sp., all of which emerge from the cocoons. 



