2. _Dentate_, when the joints are armed with slight pointed spines.
{ xi}3. _Serrate_, when the joints have sharp projections like the teeth of a saw. (Fig. 18, antenna of _Melanchra composita_.)
4. _Filiform_, when the whole antenna is simple or thread-like. (Fig. 19, antenna of _Epirranthis alectoraria_.)
The clothing of the antennæ also varies, and is distinguished as under:--
1. _Ciliated_, when clothed with one or two series of short, fine hairs.
2. _Fasciculate-ciliated_, when the hairs are collected into tufts. (Fig. 17, antenna of _Chloroclystis plinthina_.)
3. _Pubescent_, when the antennæ are clothed with uniform short hairs. (Fig. 19.)
The functions of the antennæ are still a matter of dispute amongst entomologists. The majority of the older naturalists regarded them as organs of hearing. The antennæ are almost always more fully developed in the male than in the female. From this circumstance many modern entomologists consider that one of their functions is to enable the former to find the latter.
The organs of the mouth are thus distinguished:--
1. The _Labrum_, or upper lip (Plate I., fig. 11, _l_), a minute rudimentary plate situated in front immediately above the proboscis.
2. The _Mandibles_, or upper jaws (m.m), two minute sickle-shaped organs situated just below the labrum, also rudimentary.
3. The _Proboscis_, or _Haustellum_[1] (c), a tubular extensible organ formed of the two maxillæ, or lower jaws, which have become greatly elongated, semi-tubular, and closely pressed together at the edges, but separable at the will of the insect--a structure which enables the organ to be easily cleansed when necessary, and is extremely interesting as indicating so clearly the true development of the proboscis from the maxillæ.
The _Maxillary palpi_ (p.p) are two jointed organs attached to the base of the proboscis and very frequently rudimentary, but fully developed amongst certain of the _Micro-Lepidoptera_.
The _Labium_, or lower lip, is situated below the proboscis and carries the _Labial palpi_ (figs. 5 and 6), two large jointed organs which are very conspicuous in nearly all the species and often quite conceal the maxillary palpi. They are usually regarded as organs of touch, but their true function does not seem to be properly understood. In the _Lepidoptera_ they appear to protect the proboscis, which, when out of use, is always coiled up in a spiral between them. The labrum and mandibles can only be seen by removing the large labial palpi.