3. Specialised Forms.—The apple mussel-scale ([Figs. 5] and [7], Nos. 2 and 6) is a representative of this group, the members of which are markedly specialised, the legs and antennæ of the adult female becoming completely atrophied during development, and the shape of the body profoundly altered; protection is afforded by a scale-like covering not attached to the body. In the mussel-scale development there are two larval stages: the first, like all coccids, has the legs and antennæ well developed and is active.
FIG. 5.—ILLUSTRATIONS OF DIFFERENT TYPES OF
SCALE-INSECT LIFE-HISTORIES.
On settling to feed, this first larva commences to produce a covering of white threads that mat together to form the first scale; the second stage larva presents profound changes in the absence of legs and antennæ, while the body has become pear-shaped, the head, thorax and abdomen seeming as one; a second more waxy scale is now formed. After a second moult, the adult appears, and resembles the second stage larva in form; the adult constructs a third scale, very much larger than the earlier ones, to which it remains attached by its anterior end.
Though many of the specialised coccids form elongate scales, as in the case of the mussel-scale, numerous others construct circular scales, as does the San José ([Fig. 5]); in the latter, the second and third scales are constructed round the first, so that the first and second appear as pimple-like structures in the centre, or slightly to one side of the completed covering. As with the olive scale, the covering of the specialised forms serves as a receptacle for the eggs ([Fig. 5]).
Some of the more important coccids occurring in New Zealand will now be discussed.
Cottony Cushion Scale (Icerya purchasi).—This insect ([Fig. 6]a) is a native of Australia, but has now become established in many other countries, including New Zealand. For a time it was a serious pest of citrus, until the introduction and establishment of its natural enemy, the ladybird beetle (Novius cardinalis).
The adult female is more or less oval, and covered with a yellowish powder, partly concealing the reddish-brown ground colour and dark spots along the sides of the body; the legs are black. A characteristic feature is the white corrugated egg-sac attached to the end of the body ([Fig. 5]). As the eggs are laid, this sac increases in size, until it may measure fully 2 ½ times the length of the parent, which becomes tilted up. The eggs are orange-yellow, and as many as 800 may be produced by a single female. The eggs hatch in about a fortnight during summer, and the period of development to the adult ranges from three to five months. The larvæ most frequently congregate along the mid-ribs of leaves, and as development advances they usually migrate to the twigs and branches. There are two generations each year. A considerable variety of plants is attacked by this insect, chief among which are citrus, acacia, gorse, wattle, and Douglas fir.