26 PRINCIPAL CACTUS INSECTS OF UNITED STATES. 



is shown in one of the accompanying photographs. (PL VII. fig. 1.) 



The individual egg masses may contain as many as 30 eggs. 



There seems to he only one brood each year. As soon as the larvae 

 hatch in the spring they begin feeding upon the surface of the joint. 

 Within a few days they make their way to the inside and never 

 appear upon the surface. The experience of all observers is that only 

 one or two larvae are ever found within a joint. This is remarkable 

 in view of the fact that the eggs are deposited in such numbers. 

 Apparently it is not a case of the young larvae traveling from one 

 joint to another, since frequently only one or two joints on a plant 

 are found to be infested. Undoubtedl} 7 the larva 1 are cannibalistic in 

 habits, and this accounts partly for the fact that these isolated indi- 

 viduals are found; but there is also another factor to be considered. 

 The work of the larvae immediately causes a strong reaction on the 

 part of the plant. A copious secretion of proliferous tissue is formed 

 and larvae have been frequently found completely engulfed in this 

 formation. Undoubtedly the pressure frequently results in the 

 destruction of the larvae. 



Although this species is an internal feeder, the indications of its 

 work are more or less conspicuous. The joint soon takes on a yel- 

 lowish appearance and the large swellings on both sides of the joints 

 are common sights in the cactus countiy! The entire interior is 

 destroyed and the proliferous growth causes the swellings which 

 frequently result in the increase in the thickness of a joint by three 

 or four fold. Strangely such swollen joints are sometimes found to 

 contain no larvae. The evidence of their work is always present. 

 Pressure from the proliferous growth ma} 7 have caused the death of 

 the larvae in such cases. 



The effect upon the plant is generally to cause the death of the 

 joint or joints which are infested. The injury is made greater by a 

 number of scavengers, principally dipterous. As the larva 1 fre- 

 quently make their way through the stem from one joint to another, 

 it is not uncommon for several joints to be killed outright. Of course 

 the portion of the plant above the infested joints dies from lack of 

 nutrition. After a time the wind causes the diseased branch to fall 

 to the ground. In case the larvae are killed by pressure the swelling 

 subsides. The sides, however, do not unite and the joint remains 

 deformed. Mr. J. D. Mitchell, who has made many careful observa- 

 tions on this species, believes that the partial healing of the injury 

 follows when the exit is at the lowest pari of the stem, and that the 

 joint falls invariably when the exit is near the top and the softened 

 excrement and proliferous tissue can not escape. 



Although this insect is not extremely abundant in any locality 

 where observations have been made, it is to be found throughout the 

 cactus area. In some localities at least one plant in every clump lias 



