Wasps 



Life History of a Parasitic Wasp 



(Lcelius trogodermatis Ashm.^ 



When Mr. Busck was making some studies for me in the 

 summer of 1897, on the life history of the tussock moth, he made 

 the interesting discovery that the egg masses of this famous shade- 

 tree defoliator were sometimes eaten by the larvae of certain der- 

 mestid beetles which are ordinarily known as museum pests, 

 feeding upon skins and furs, stuffed birds and pinned insects, and 

 which, although known to feed upon dead and dry animal matter, 

 were hardly to be suspected of eating living animal matter. But 

 we found (and this is by the way) that these museum pests were 

 really destroying the sound eggs of the tussock moth. This in- 

 terested us so much that egg masses with dermestid larvae were 

 brought into the insectary for most careful observation. Then it 

 was found that with the dermestids had also been brought in a 

 most interesting parasite which proved to be Lcelius trogoderma- 

 tis a Bethylid. The Laelius is a little, black, slender, active, four- 

 winged fly; and the female, when it finds one of these dermestid 

 larvae, at once jumps upon its back and clings firmly, in spite of 

 the struggles of the victim. As soon as the poor beetle larva quiets 

 down a bit, Laelius places herself crosswise over the thorax and, 

 curling her abdomen around under the side, inserts her sting just 

 behind the second or third pair of legs, paralyzing the dermestid 

 instantly, the sting apparently having entered one of the large 

 thoracic nerve ganglia. Then the parasite relaxes its hold and 

 begins pulling the legs and hairs of its victim with its mandibles, 

 its antennae vibrating in a contented manner. The pulling of the 

 legs is evidently an attempt on the part of the parasite to see if 

 the stinging has done its work with perfect effect. Having satis- 

 fied herself by all sorts of tests that the paralysis is complete, she 

 proceeds to lay an egg, attaching it to the skin of the dermestid 

 on the under side of the body, first pulling out the hairs carefully 

 so that the egg can be firmly attached to the skin. If in the course 

 of this operation, or even before the egg is laid, another dermes- 

 tid larva comes within her range of perception, she leaves the 

 first victim, mounts and stings the second, or even a third or a 

 fourth, each time testing the completeness of the paralysis with 

 the utmost care. Before attaching the egg she thrusts her sting 

 into the spot several times, apparently making an orifice through 



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