13 



TOBACCO-WORM. A DESTnOYEK OP THE PARASITE DISCOVERED. 



On one occasion, wlien I was contemplating one of the tobacco-worms 

 which I met with covered over with parasitic cocoons, I noticed a very 

 email fly wandering about among the cocoons. My first thought was that 

 this fly was probably one of the Microgaster parasites which had just then 

 come from some one of these cocoons ; but the query soon arose in my mind, 

 ■whether it might not be an enemy, stinging the cocoons to destroy their 

 inmates in the same manner they had destroj'ed the tobacco-worm. Its 

 very small size did not enable the eye to discover whether it really was 

 one of the Microgaster flies. I was so fortunate as to succeed in enclosing 

 it in a small vial, and then upon examining it with a magnifier, I became 

 assured it had not come from the cocoons, for I perceived it pertained to a 

 different group of parasites from that to which the Microgaster genus 

 belongs. But how could the higiily interesting and important point be 

 ascertained, whether it actually was a destroyer of the inmates of these 

 cocoons? With the hope of obtaining further light upon this subject a 

 portion of the stalk of the plant with the tobacco-worm adhering to it 

 was cut off and enclosed in a glass jar. On the fifth day thereafter, two 

 Microgaster flies made their appearance in the jar, and the worm now 

 being dead and beginning to become putrid, the cocoons were all removed 

 from its surface and enclosed in a vial. It was feared that this slight vio- 

 lence to them had destroyed their inmates, as day after day now elapsed 

 and no more flies came from them. But, three months later, in December, 

 they being kept in a warm room, a dozen flies were discovered, wandering 

 around in this vial ; and for some weeks after, others continued to come 

 forth from the cocoons. And these proved to be identical with the single 

 fly which had been captured among these cocoons so long a lime before. 

 It was therefore evident that that fly was the parent of these which were 

 now issuing from the cocoons ; and so industrious had that little creature 

 been, that it had punctured and dropped one of its eggs into all save two 

 of the cocoons, which were more than a hundred in number ; and these 

 two, it is probable, would not have escaped, if the fly had not been inter- 

 rupted and taken away from its work. 



. These destro^'ers of the insect which destroys the tobacco-worm arc very 

 small four-winged flies of a shining dark green color, with pale yellowish 

 legs and white feet. They belong to the order Hymekoptera and the family 

 Chalcidid^, and are closely related to the Hessian fly parasite, Semiotel- 

 lus destructor, figured in my Seventh Report, plate 3, fig. 1, which figure 

 •will also serve to represent this insect in almost every particular. It per- 

 tains to the genus Pleromalus, a name derived from two Greek words, 

 meaning bad wings, the wings in these insects being nearly destitute of 

 ribs or veins. As they, by destroying the parasite of the tobacco-worm, 

 cause that worm to be more numerous and hereby more injurious to the 

 tobacco, and as they will often occur lurking about this plant in search of the 

 cocoons upon which to bestow their eggs, thej' may not inappropriately 

 be named the Tobacco Pteromalus. All the flies which came from the 

 cocoons were females, from which the following description is drawn. 

 The Tobacco Pteromalus (Pleromalus Tahacum),\B one-tentU of an inch 



