tridulating organs just
described is presumably to afford means of recognition by sound. Some
beetles emit a bright light from a portion of their bodies, which leads
to the recognition of mate or comrade by sight. In the wingless female
glow-worm (_Lampyris_, fig. 15 f) the luminous region is at the hinder
end, the organ emitting the light consisting, according to H. von
Wielowiejski (1882), of cells similar to those of the fat-body,
containing a substance that undergoes oxidation. The illumination is
intermittent, and appears to be under the control of the insect's
nervous system. The well-known "fire-flies" of the tropics are large
click-beetles (_Elateridae_), that emit light from paired spots on the
prothorax and from the base of the ventral abdominal region. The
luminous organs of these beetles consist of a specialized part of the
fat-body, with an inner opaque and an outer transparent layer. Its
structure has been described by C. Heinemann, and its physiology by R.
Dubois (1886), who considers that the luminosity is due to the influence
of an enzyme in the cells of the organ upon a special substance in the
blood. The eggs and larvae of the fire-flies are luminous as well as the
perfect beetles.

_Fossil History._--The Coleoptera can be traced back farther in time
than any other order of insects with complete transformations, if the
structures that have been described from the Carboniferous rocks of
Germany are really elytra. In the Triassic rocks of Switzerland remains
of weevils (_Curculionidae_) occur, a family which is considered by many
students the most specialized of the order. And when we know that the
_Chrysomelidae_ and _Buprestidae_ also lived in Triassic, and the
_Carabidae_, _Elateridae_, _Cerambycidae_ and _Scarabaeidae_, in Liassic
times, we cannot doubt that the great majority of our existing families
had already been differentiated at the beginning of the Mesozoic epoch.
Coming to the Tertiary we find the Oligocene beds of Aix, of east
Prussia (amber) and of Colorado, and the Miocene of Bavaria, especially
rich in remains of beetles, most of which can be referred to existing
genera.

_Classification._--The Coleoptera have been probably more assiduously
studied by systematic naturalists than any other order of insects. The
number of described species can now hardly be less than 100,000, but
there is little agreement as to the main principles of a natural
classification. About eighty-five families