Vill INTRODUCTION. 
which children are so fond, emit a similar, though not quite so 
offensive an odour. The rove-beetles (Staphylinidz), in addition 
to their threatening and formidable attitudes, emit a very dis- 
agreeable odour, though it is not quite so bad as that of others 
(Silphidee) which feed on carrion. The “church-yard beetle” 
(Blaps mortisaga) has been noted for the same circumstance since 
the time of Pliny (Hist. Nat. xxix. 6). Some bees (Andrenide) 
have a strong smell of garlic, which may probably be disagreeable 
to their various enemies (Kirby, ‘ Monographia,’ 1. p. 136). The 
very beautiful caterpillar of the swallow-tailed butterfly (Papilio 
Machaon, Linn.) possesses a curious organ supposed to be intended 
for the similar purpose of defence ; it is of a dark orange colour, 
and is always concealed within one of the black rings on the 
shoulders, unless the creature be irritated, when it darts it out to 
the extent of about an inch and at the same time emits a strong 
odour resembling fennel. This may be intended to intimidate 
the ichneumons from depositing their parasite eggs in its body, 
or to warn off the thrushes or other creatures from devouring it. 
Although the sense of smel] in man is very inferior to that 
developed in animals yet it is marvellously delicate, as some recent 
experiments made by Professor Vallentine prove :—He found that 
a current of air containing of a milligramme of oil of rose 
mie 
could be perceived by the sense of smell. He also ascertained 
that the amount of odoriferous air which must pass over the 
olfactory membrane in order to excite the sense of smell was from 
;|, to + of a pint ; he calculated, therefore, that the actual amount 
of oil of rose necessary to excite the sense of smell was about 
—~. of a grain. The power of perceiving the presence of 
zoldt of Erlangen have found that air containing Ear = of a 
000,000 
milligramme of this compound to the cubic centimétre of air could 
: ee 1 
be appreciated, and it was estimated that only 460,000,000 of a 
milligramme of mercaptan is necessary to excite a sensation of 
smell. 
The pleasant odour of the soil has been traced by Berthelot, the 
