The _Hydropsyches_ (Fig. 421) and _Rhyacophili_ (Fig. 422) are small insects which resemble the _Phryganeæ_ very closely. Their larvæ have, for the purposes of respiration, some gills, others retractile tubes. They construct for themselves fixed places of shelter, more or less imperfect, at the bottom of the water, and against large stones, which they leave occasionally for a few moments. Sometimes these cases contain several larvæ. Fig. 421 represents the various states of a _Hydropsyche_; the larva is seen on the left, the pupa on the right, the winged insect in the middle. Two of the insect's tents, or places of shelter, are represented below. Fig. 422 shows the different states of _Rhyacophilus vulgatus_, larva, cocoon, pupa, and imago. The genus _Rhyacophilus_ has this peculiarity, that the larva spins itself a cocoon in the interior of its dwelling before changing into a pupa. [Illustration: Fig. 421.--Hydropsyche (Phryganea) atomaria, larva, pupa, imago, and larva-case.] [Illustration: Fig. 422.--Rhyacophilus vulgatus, larva, pupa, cocoon, and imago (male).] IX. COLEOPTERA. In collections of insects the Coleoptera almost always occupy the principal place. They are sought after by collectors on account of the brightness of their colours, of the solidity of their integuments, and the facility with which they can be preserved. This circumstance has contributed much to give to the Coleopterous Order marked preponderance in the immense series of insects. Many more have been collected than any one has as yet been enabled to describe; and the collections are encumbered with species of which no naturalist has yet given an account. Admitting that the first-rate collections contain each about 25,000 perfectly distinct species, and that a certain fraction of these treasures is peculiar to each collection, M. Blanchard came to the conclusion that we must estimate the number at more than 100,000 of the species of Coleoptera which would be obtained if the different entomological collections of France, England, and Germany were put together. But every day we see arriving from different regions of the globe new riches, hardly dreamt of up to that time; and it is not only the small species, but the larger and more beautiful also, which furnish their contingent. It may, then, be believed that, if the entire surface of the earth were carefully explored, we should obtain an incalculable number of Coleoptera, having sufficient characteristics to constitute distinct species or kinds.