Fabricius and Sprengel state that when flies have once entered the flowers of Aristolochia they never escape,—­a statement which I could not believe, as in this case the insects would not aid in the cross-fertilisation of the plant; and this statement has now been shown by Hildebrand to be erroneous.  As the spathes of Arum maculatum are furnished with filaments apparently adapted to prevent the exit of insects, they resemble in this respect the flowers of Aristolochia; and on examining several spathes, from thirty to sixty minute Diptera belonging to three species were found in some of them; and many of these insects were lying dead at the bottom, as if they had been permanently entrapped.  In order to discover whether the living ones could escape and carry pollen to another plant, I tied in the spring of 1842 a fine muslin bag tightly round a spathe; and on returning in an hour’s time several little flies were crawling about on the inner surface of the bag.  I then gathered a spathe and breathed hard into it; several flies soon crawled out, and all without exception were dusted with arum pollen.  These flies quickly flew away, and I distinctly saw three of them fly to another plant about a yard off; they alighted on the inner or concave surface of the spathe, and suddenly flew down into the flower.  I then opened this flower, and although not a single anther had burst, several grains of pollen were lying at the bottom, which must have been brought from another plant by one of these flies or by some other insect.  In another flower little flies were crawling about, and I saw them leave pollen on the stigmas.

I do not know whether Lepidoptera generally keep to the flowers of the same species; but I once observed many minute moths (I believe Lampronia (Tinea) calthella) apparently eating the pollen of Mercurialis annua, and they had the whole front of their bodies covered with pollen.  I then went to a female plant some yards off, and saw in the course of fifteen minutes three of these moths alight on the stigmas.  Lepidoptera are probably often induced to frequent the flowers of the same species, whenever these are provided with a long and narrow nectary, as in this case other insects cannot suck the nectar, which will thus be preserved for those having an elongated proboscis.  No doubt the Yucca moth visits only the flowers whence its name is derived, for a most wonderful instinct guides this moth to place pollen on the stigma, so that the ovules may be developed on which the larvae feed. (11/4.  Described by Mr. Riley in the ‘American Naturalist’ volume 7 October 1873.)With respect to Coleoptera, I have seen Meligethes covered with pollen flying from flower to flower of the same species; and this must often occur, as, according to M. Brisout, ’many of the species affect only one kind of plant.” (11/5.  As quoted in ‘American Nat.’  May 1873 page 270.)