530 HYMENOPTERA. 



are placed, singly, within little semicircular incisions through 

 the skin of the leaf, and generally on the lower side of it. 

 The flies have not the timidity of many other insects, and 

 are not easily disturbed while laying their eggs. On the 



fourteenth day afterwards, the eggs begin to hatch, 

 ^ and the young slug-worms (Fig. 249) continue 



to come forth from the 5th of June to the 20th 

 of July, according as the flies have appeared early or late 

 in the spring. 



At first the slugs are white ; but a slimy matter soon 

 oozes out of their skin and covers their backs with an olive- 

 colored sticky coat. They have twenty very short legs, or 

 a pair under each segment of the body except the fourth 

 and the last. The largest slugs are about nine twentieths 

 cif an inch in length, when fully grown. The head, of a 

 dark chestnut color, is small, and is entirely concealed under 

 the fore part of the body. They are largest before, and 

 taper behind, and in form somewhat resemble minute tad- 

 poles. They have the faculty of swelling out the fore part 

 of the body, and generally rest with the tail a little turned 

 up. These disgusting slugs live mostly on the upper side 

 of the leaves of the pear and cherry trees, and eat away 

 the substance thereof, leaving only the veins and the skin 

 beneath untouched. Sometimes twenty or thirty of them 

 may be seen on a single leaf; and in the year 1797 they 

 were so abundant, in some parts of Massachusetts, that 

 small trees were covered with them, and the foliage en- 

 tirely destroyed; and even the air, by passing through the 

 trees, became charged with a very disagreeable and sick- 

 ening odor, given out by these slimy creatures. The trees 

 attacked by them are forced to throw out new leaves, dur- 

 ing the heat of the summer, at the ends of the twigs and 

 branches that still remain alive ; and this unseasonable fo- 

 liage, which should not have appeared till the next spring, 

 exhausts the vigor of the trees, and cuts off the prospect 

 of fruit. 



