39 



ful, that whilst every kind of tree and plant appears to have one or more species of aphis in- 

 festing and blighting it, each species of aphis seems to have a particular parasite preying upon 

 and devouring it ; for each kind of aphis, from which I have reared these insects, has fur- 

 nished a species differing from all the others, and, in some instances, two species have been 

 obtained from one kind of aphis." 



How complicated and how wonderful are the marvels of Nature. There seems a provi- 

 dence in all these arrangements. Kirby has well said that it is strange that among the in- 

 numerable species of insects, many of them e.vtremely fragile and exposed to dangers and 

 enemies without end, no link should be lost from the chain, but all be maintained in those 

 relative proportions necessary for the general good of the system ; that if one species for a 

 while preponderate and instead of preserving seem to destroy, yet counterchecks should at 

 the same time be provided to reduce it within its due limits ; and further, that the operations 

 of in.sects should be so directed and overruled as to effect the purposes for which they were 

 created, and never exceed their commission ; nothing can furnish a stronger proof than this, 

 that an unseen hand holds the reins, now permitting one to prevail and now another, as shall 

 best promote certain wise ends, and saying to each " Hitherto shn\t thou come and no further." 



A few words will suffice to indicate the remedies which man has devised to lessen the 

 numbers of these destroyers of vegetation. It is universally conceded, that where the remedy 

 can be conveniently applied there is nothing more effectual than tobacco smoke. To exter- 

 minate them in green-houses, smoking — by strewing a few leaves of tobacco on hot coals — is 

 regularly resorted to, and, if thoroughly done, is very effectual. A few favourite plants may 

 bo similarly treated by enclosing them in a light paper-covered frame and smoking under it, 

 or by placing them under an inverted barrel or box ; after such treatment the plants should 

 be immediately washed with luke-warm water from a syringe or watering-pot. Where this 

 remedy cannot be applied, drenching with tobacco-water has been recommended, also the ap- 

 plication of strong soap-suds, or weak lye, sprinkling it freely on the plant, and even dipping 

 the succulent ends, where the aphides cluster, into the liquid. 



Hot water has also been recommended, but this, if not cautiously applied, is very apt to 

 injure the plant. Some species will bear an application of water heated to 130° Fahrenheit; 

 indeed, some few will bear a higher temperature than this without injury, but others are more 

 susceptible in this respect, hence the remedy requires careful handling. 



