agments of wood, sawdust, etc.; anything is
good. These Hymenoptera possess no organ specially adapted to aid
them; it is with their saliva that they glue this dust together and
make of it a substance very suitable for its purpose. The dwellings
often reach considerable size, yet they are always begun by a single
female, who does all the work without help until the moment when the
first eggs come out; she is thus furnished with workers capable of
taking a share in her task. The _Vespa sylvestris_ builds a paper nest
of this kind, hanging to the branch of a tree, like a great grey
sphere prolonged to a blunt neck. (Fig. 33.) The Hornet's nest is
similar in construction.

_Gelatine nests._--These are made by certain Swallows who nest in
grottoes or cliffs on the edge of the sea. After having collected from
the water a gelatinous substance formed either of the spawn of fish or
the eggs of Mollusca, they carry this substance on to a perpendicular
wall, and apply it to form an arc of a circle. This first deposit
being dry, they increase it by sticking on to its edge a new deposit.
Gradually the dwelling takes on the appearance of a cup and receives
the workers' eggs. (Fig. 34.) These dwellings are the famous swallows'
nests, so appreciated by the epicures of the extreme East, which are
edible in the same way as, for example, caviare.

_Constructions built of earth--Solitary masons._--Certain animals,
whose dwelling participates in the nature of a hollow cavern, make
additions to it which claim a place among the constructions with which
we are now occupied.

[Illustration: FIG. 34.]

[Illustration: FIG. 35.]

The _Anthophora parietina_ is in this group; it is a small bee which
lives in liberty in our climate. As its name indicates, it prefers to
frequent the walls of old buildings and finds a refuge in the
interstices, hollowing out the mortar half disintegrated by time. The
entrance to the dwelling is protected by a tube curved towards the
bottom, and making an external prominence. (Fig. 35.) The owner comes
and goes by this passage, and as it is curved towards the earth the
interior is protected against a flow of rain, while at the same time
the entry is rendered more difficult for _Melectes_ and _Anthrax_.
These insects, in fact, watch the departure of the _Anthophora_ to
endeavour to penetrate into their nests and lay their eggs there. The
gallery of entry and exit has been built with grains of sand, the
_debris_ p