214 REPORT UPON COTTON INSECTS. 



Didictyum zigzag sp., 32 specimens ; of the small Dipteron Phora aletiae, 

 a very great number of specimens ; making altogether of the large 

 parasites 44, each singly from a chrysalis, and 120 chrysalides destroyed 

 by the small parasites, making a total of 104 out of 1,721, or between 9 

 and 10 per cent. The remaining 102 died from some unknown cause. 

 This percentage is small, but in the last brood it would unoubtedly 

 be greater. 



The extravagant ideas of Dr. Gorham on the subject of the extent of 

 parasitism are easily accounted for. He collected his specimen chrysa- 

 lides for observation late in the fall, after the hibernating moths had 

 issued. Naturally, no apparently sound chrysalides were left excepting 

 those containing parasites. These he collected, and parasites issued 

 from all; hence his conclusions. A little note from one of Professor 

 Willet's letters seems to indicate the greater abundance of parasites 

 in the last brood than in the earlier ones. He collected a number of 

 newly-formed chrysalides in November. Of these he says : 



About two dozen were placed in a box in my sitting-room, expecting to hatch out 

 some moths for exposure. The following is the result : In some two weeks two moths 

 came out; they seemed delicate, and one lived only two days, the other four or five. 

 No other moths have appeared (December tl). November 24, 1 found four ichneumon 

 flies (Pimpla conquUitor) out in one boll ; December 2, one more, and December 7 an- 

 other ; the sixth, the last, with no ovipositor (a male). In breaking open the dried 

 chrysalides I destroyed two pupae of parasites. These make eight parasites in some 

 two dozen chrysalides a large proportion. I had 75 chrysalides in a box in summer ; 

 about 50 came out moths ; most of the others could not escape from and perished in 

 the dried leaves. I saw not a parasite of any kind. 



An encouraging statement concerning the extensive parasitism of an 

 early brood (the third) is contained in a letter from Mr. Trelease of July 

 24, 1879. He stated that at that time nearly one-half of the half-grown 

 worms in the fields under his observation bore the eggs of one of the 

 Tachina parasites. One-half is certainly a large proportion, but he re- 

 iterates it with exactness in his notes, and stands ready to vouch for it. 

 It seems not at all unlikely when Ave consider the numbers in which the 

 northern species of Tachina occur in fields ravaged by the northern army- 

 worm. In a field which was black with these worms I have searched 

 for hours without finding a single unparasitized full-grown worm. Nine 

 hundred and ninety-nine out of a thousand bore the white eggs of the 

 destroyer. 



These few points will be sufficient, perhaps, to give a more accurate 

 idea of the importance of the natural enemies. 



