1 46 On the Hessian Fly. 



slieath, and continues its course until it reaches the bot- 

 tom of the sheath ; here it fixes to the stalk, if shot up, 

 if not, to the head of the plant where the pedicle of the 

 leaf joins, with its head downwards, and with its teeth 

 it perforates into a sap vessel, from which it draws its 

 nourishment. Having thus placed itself in a proper si- 

 tuation, it never alters its position, until it under^^oes the 

 remaining transformations. By the sap it is nourished, 

 its body enlarges, loses its wrinkled appearance, changes 

 to a paler colour, becomes plump and hard, so as to re- 

 sist the pressure of the stalk and sheath, and as it daily 

 increases in bulk, it presses with such force against the 

 stalk as to print its shape in it, and thus prevents the sap 

 from passing the side of the stalk against which it is 

 fixed. One insect thus placed seldom destroys the plant, 

 but when two or three are fixed in this manner round the 

 stalk or head of a plant, they certainly destroy it ; not by 

 the nourishm.ent they draw from the sap, or eating any 

 part of the plant, but by the pressure they make, stopping 

 the ascent of sap, and thus cutting off the nourish- 

 ment of the plant. They are frequently found in the 

 second sheath from the ground, after the stalk is shot 

 up ; but when this is the case, the eggs have been depo- 

 sited late in the season, and the stalk has acquired such a 

 degree of hardness before the caterpillar begins to press 

 against it, that very little impression is made on the stalk, 

 and litde or no damage sustained. And here it may be 

 worth our notice to see how unerring instinct operates 

 on these little animals, scarce visible to the naked eye. 

 The minute they are excluded from the eggs, they are 

 directed to a proper situation, and invariably place them- 

 selves with their heads to the root of the plant, that their 

 nourishment may not be cut off in stopping the ascent of 

 the sap, whereby they would perish, as happens to those 

 that are placed uppermost on the stalk when a great num- 



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