68 THE POPULAR SCIENCE MONTHLY. 



known to the laundress, and to all who have seen cooked arrowroot. 

 If this paste be dried by evaporation, it does not regain its former in- 

 solubility, but readily dissolves in hot or cold water. This is what I 

 should describe as cooked starch. 



Starch may be roasted as well as boiled, but with very different 

 effects. The changes that then occur are much more decided, and 

 very interesting. I will describe them in my next. — Knowledge. 



■♦»» 



HOW FLIES HANG OK 



By Dr. J. E. EOMBOUTS. 



IT was believed at one time that flies and some other insects owe 

 the faculty of running over smooth bodies like glass to the nu- 

 merous hairs with which their feet are provided catching in the pores 

 of the material. The absurdity of this supposition is readily apparent 

 on examining glass with the microscope ; and no naturalist can be 

 found in these days to uphold it. Another theory, which has been 

 frequently advanced, explains the fact by affirming that the feet ter- 

 minate in little suckers, by the application of which to the smooth sur- 

 face the insect is able to adhere by the force of the pressure of the 

 air, in the same manner that the street-boy fastens his leather sucker 

 tightly to the flagging. Blackwall's investigations have demonstrated 

 that such a contact as is here supposed does not take place. He has 

 seen flies running over the inner sides of the bell-wlass receiver of an 

 air-pump from which the air had been exhausted. If we examine the 

 foot of a fly through the microscope, we shall fine! that there are no 

 suckers on it, but that the foot-cushions are furnished with very fine 

 hairs that prevent all close contact with the glass. The theory in ques- 

 tion which invokes the pressure of the air was first broached by Dr. 

 Derham, and was accepted by most of his contemporary entomologists. 

 Other observers, among them Dr. Hooke, were of the opinion that the 

 insects were able to attach themselves to the glass by virtue of some 

 sticky matter in or on the hair. Blackwall explained the fact by say- 

 ing that a viscous substance flowed from each hair ; and probably the 

 majority of the later entomologists have accepted this explanation. In 

 answer to it, we may say that, if there really were a flow of a viscous 

 fluid from the hairs, the flies would not be able to move after they had 

 rested in one spot for a little while, for the liquid would have dried or 

 hardened so as to detain them ; but we know that the insect can always 

 fly away instantaneously, even if it has remained in the same place for 

 hours without moving, 



I have concluded from my experiments that it is not the pressure 

 of the air nor the power of an adhesive liquid that gives flies the fac- 



