270 NATURAL HISTORY 



imagine that these beginnings were intentionally made 

 in order to be in the greater forwardness for next spring, 

 is allowing perhaps too much foresight and rerumpru- 

 dentia to a simple bird. May not the cause of these 

 latebrce being left unfinished arise from their meeting in 

 those places with strata too harsh, hard, and solid, for 

 their purpose, which they relinquish, and go to a fresh 

 spot that works more freely ? Or may they not in other 

 places fall in with a soil as much too loose and mould- 

 ering, liable to founder, and threatening to overwhelm 

 them and their labours ? 



One thing is remarkable that, after some years, the 

 old holes are forsaken and new ones bored; perhaps 

 because the old habitations grow foul and fetid from 

 long use, or because they may so abound with fleas as 

 to become untenantable. This species of swallow more- 

 over is strangely annoyed with fleas : and we have seen 

 fleas, bed fleas (Pulex irritans 1 ), swarming at the mouths 

 of these holes, like bees on the stools of their hives. 



The following circumstance should by no means be 

 omitted that these birds do not make use of their 



1 The flea of the sand martin, although to the unassisted eye so exceed- 

 ingly similar to the bed flea as to be scarcely distinguishable from it, is 

 altogether distinct. It appears even to be distinct from the flea of the 

 swallow, named by Mr. Stephens Pulex hinmdinis ; and has been indi- 

 cated by Mr. Curtis under the appellation of btfasciatus. By the latter 

 the sand martin's flea is referred to a genus separated by him from the 

 ordinary flea, Pulex, LINN., and distinguished by the name of Cerate- 

 phyllus : he having discovered that the antennae of the numerous insects 

 referrible to this last-named group have four or more joints; while in 

 Pulex irritans and its congeners those organs are only two-jointed. 



Although it was stated by Latreille, so long since as the date of the 

 publication of his Genera Crustaceorum et Insectorum, that fleas possess 

 antennae, which he described as being situated on each side in a cavity 

 behind the eye, the minuteness of these little creatures rendering their 

 examination difficult, obstructed until about three years since the verifi- 

 cation of the fact by others : Latreille himself appearing, in 1829, to have 

 hesitated in averring it with the same certainty that he had expressed 

 upwards of twenty years before. In 1832, however, Mr. Haliday and 

 Mr. Curtis in England, and later in the year, M. Duges in France, redis- 

 covered these organs : and figures of them, as they were observed in se- 

 veral species, were given in the British Entomology and in the Annalcs 



