i8 



AN ECONOMIC ENTOMOLOGY. 



Fig. 5- 



a rule, they will bear close watching. One thing is certain, how- 

 ever : no insect with this type of mouth structure can be reached 

 by means of a stomach poison. Destruction through its food is, 



therefore, absolutely impossi- 

 ble, because we cannot poison 

 the plant juices by any method 

 thus far at our command. An 

 insect of this kind is incapable 

 of eating any solid food what- 

 ever, and no matter how thor- 

 oughly covered with a corro- 

 sive poison the outside of our 

 plants may be, it would get 

 none, since it only punctures 

 the tissue without absorbing 

 any portion of the outer sur- 

 face. Thus the determination 

 of the type of mouth structure 

 often limits or decides the char- 

 acter of the remedy to be used 

 in destroying the insect. 



Among the Diptera, or flies, 

 we have a number of interest- 

 Mouth parts of a plant-louse.— a, the ing modifications of the suck- 



jointed beak ; the lancets, much enlarged ; Jj^^r niouth. One SCrieS is fur- 

 c and d illustrate the feeler and foot. , . * , 



nished with lancets similar to 

 those in the bugs, but more numerous, representing different 

 mouth structures, and not always carried into the head itself. 

 The sucking structure is also quite different, and never forms a 

 rigid, jointed beak. Mosquitoes and horse-flies are examples of 

 this kind ; but gradually the lancets disappear, and in most of the 

 flies only the sucking lip, often greatly and interestingly developed, 

 remains. The common house-flies and blow-flies are types of this 

 modified form, and are capable of taking liquid food only, though 

 often seen attacking soHds. If the mouth of a fly be examined 

 under the microscope, there will be seen at the tip of the lip a 

 series of deep grooves, stiffened with chitinous loops, and armed 

 with sharp projecting edges. When the fly wishes to feed on a 

 solid, it scrapes the surface by means of these rasp-like projec- 



