174 NATURE'S TEACHINGS. 



compensates for them by preserving the intrusive Ichneumon- 

 fly, and giving in his diary full details of the insect on which 

 it was parasitic, of the plant on which the caterpillar lived, the 

 date of its appearance, and its numbers. 



Now, there are many of these parasitic insects, notably those 

 belonging to the genus Microgaster, which invariably make 

 doors in their cocoons. I have now before me groups of 

 cocoons made of the two commonest British species, namely, 

 Microgaster glomeratus and Microgaster akearim, and in both 

 of them each tiny cocoon is furnished with a hemispherical, 

 hinged door. I have also some exquisitely beautiful groups 

 of Microgaster cocoons found in the West Indies. They 

 are the purest white, shine with a satiny lustre, and are 

 arranged round a hollow centre, much as if they had been 

 gummed to the outside of a very large thimble. There are 

 many hundreds of them, and every one has its little door 

 still open as it was when the fully developed insect first made 

 its escape. 



ANOTHER curious example of a natural door may be seen by 

 those who will look for it. 



On plants infested with aphides, or " green blight," as the 

 gardeners quaintly term them, may often be seen dead aphides 

 much larger than the rest, globular, brown, and shining. 

 These aphides have been " stung," as it is called, by a little 

 Ichneumon-fly belonging to the genus Ophion, and having, 

 like all its congeners, a flat and sickle-shaped abdomen. The 

 egg which has been laid in the aphis soon hatches, and the 

 young Ophion absorbs into itself all the juices of the aphis. 

 It remains within the body of its involuntary host until it is 

 fully developed, when it cuts a tiny, but beautifully perfect 

 circular door in the skin, and emerges, leaving the door open 

 and still attached by its little hinge. 



Considering the small size of the aphis, and that the dia- 

 meter of the door is only one-eighth of the length of the insect, 

 the perfection of its form is really remarkable. 



ONE of the achievements of modern Architecture is the Self- 

 closing Door, especially where it must of necessity close by its 

 own weight, and when the fitting is so exact, that even the 



