THE PRESIDENT'S ADDRESS. 221 



that the most singular facts are probably familiar to all my 

 hearers. 



Among insects, although the fleas of hairy animals are certainly 

 parasites, yet I hardly think that the flea of man can be admitted 

 into the fraternity ; but there are two little incidents relative to 

 this species which seem to me amusing. The first is that Van 

 Helmont, an early writer, seriously gives a receipt for manufac- 

 turing them, as if they were pomade or cake ; the second is that 

 Van Beneden gravely discusses whether they might not advan- 

 tageously be used instead of leeches, and whether it would be 

 worth while for that purpose to import the very large specimens 

 found at Cette and Montpellier. He appears to forget that the flea 

 produces an irritation which people are foolish enough to object to. 

 Possibly, however, he was only laughing at homoeopathy. I have 

 always thought that the Ornithomyia is a very curious parasite. It 

 is a dipterous insect, with well-developed and serviceable wings, yet 

 it lives a thoroughly parasitic life upon birds, only using its wings 

 occasionally to shift from one host to another ; but it can fly well. 

 It is said occasionally to attack man, and a nocturnal incursion 

 which a number of specimens made many years ago into the 

 military hospital at Louvain created rather a panic among the 

 inhabitants. Parasitic creatures are apt to be very imperfectly 

 developed, but in this insect not only are the claws and mouth- 

 organs powerful and complex, but also the internal organs are 

 magnificently developed. I do not know any other dipterous 

 species which makes so fine a slide for showing the optic and other 

 ganglia, and the internal organs generally. The Ornithomyia has 

 several near relatives which are not provided with wings, the 

 ordinary sheep tick (Melophagus ovis), the Lipoptena of the stag, 

 the Braula cceca of the bee, are allied creatures ; but are all 

 apterous. The Ornithomyia often exhibits a good instance of 

 hyper-parasitism ; the Anoplura and Dermaleichi of the bird 

 frequently leave it and attach themselves in considerable numbers 

 to the fly, where they appear disproportionately large. I do not 

 think that parasites of parasites are so common as some people 

 suppose. A Hypopus found on humble-bees is also found on the 

 Gamacids which infest them. These two instances may be called 

 accidental parasites, but Pagenstecher found Nematodes on the 

 Nicothoe of the lobster, and the Taenia cucumerina of the dog has 



