The _Bombus_ and _Psithyrus_ live together on the best terms, and it appears probable that the latter do the former no harm beyond appropriating a portion of their food supplies. Schmiedeknecht says they are commensals, not parasites; but it must be admitted that singularly few descriptions of the habits and life-histories of these interesting Insects have been recorded. Hoffer has, however, made a few direct observations which confirm, and at the same time make more definite, the vague ideas that have been generally prevalent among entomologists. He found and took home a nest of _Bombus variabilis_, which contained also a female of _Psithyrus campestris_, so that he was able to make observations on the two. The _Psithyrus_ was much less industrious than the _Bombus_, and only left the nest somewhat before noon, returning home again towards evening; after about a month this specimen became still more inactive, and passed entire days in the nest, occupying itself in consuming the stores of honey of its hosts, of which very large quantities were absorbed, the _Psithyrus_ being much larger than the host-bee. The cells in which the young of the _Psithyrus_ are hatched are very much larger than those of the _Bombus_, and, it may therefore be presumed, are formed by the _Psithyrus_ itself, for it can scarcely be supposed that the _Bombus_ carries its complaisance so far as to construct a cell specially adapted to the superior stature of its uninvited boarder. When a _Psithyrus_ has been for some time a regular inhabitant of a nest, the _Bombus_ take its return home from time to time as a matter of course, displaying no emotion whatever at its entry. Occasionally Hoffer tried the introduction of a _Psithyrus_ to a nest that had not previously had one as an inmate. The new arrival caused a great hubbub among the _Bombus_, which rushed to it as if to attack it, but did not do so, and the alarm soon subsided, the _Psithyrus_ taking up the position in the nest usually affected by the individuals of the species. On {61}introducing a female _Psithyrus_ to a nest of _Bombus_ in which a _Psithyrus_ was already present as an established guest, the latter asserted its rights and drove away the new comer. Hoffer also tried the experiment of placing a _Psithyrus campestris_ in the nest of _Bombus lapidarius_—a species to which it was a stranger; notwithstanding its haste to fly away, it was at once attacked by the _Bombus_, who pulled it about but did not attempt to sting it.