PRODUCTS OF THE COCCIDIDÆ.
[Waxy or cottony matter: the "honeydew" and the black fungus—"smut" or "black blight"—growing upon it.]
The Coccididæ, in some parts of the world, excrete various substances which are of commercial value, as, for example, shellac, "manna," candle-wax, &c. Cochineal is not in the same category, as it appears to be a colouring-matter pervading every cell of the tissues of the insect from which it is extracted—Coccus cacti. But there is no need to dwell here upon the ordinary excretions of the New Zealand insects, as they appear to be not sufficient either in quantity or quality for any practical service. The fibrous puparia of the Diaspidinæ appear to be quite useless. The tests of the Lecanodiaspidæ, such as Ctenochiton perforatus, Inglisia ornata, &c., although more or less waxy (but of very brittle material, often more like glass) are much too insignificant to repay any trouble taken to collect them. Of all the family, Cælostoma zælandicum, in its second stage, seems to produce the greatest amount of material, its large, hard, waxy tests being very thick and solid, and often clustered in hundreds on a root or a twig of Muhlenbeckia; but, supposing this substance (of which the true chemical nature[D] is not yet known) to be fit for some purpose, there does not seem to be any means of cultivating the insect to profit. Dactylopius alpinus produces in alcohol a rich red tint, and this not by way of excretion, but from the colouring matters of its tissues, as in the case of Coccus cacti; but here, again, the rarity of the insect and its out-of-the-way habitat would be a bar, even if nowadays it were worth while to cultivate a New Zealand cochineal. At present, therefore, there seems no reason to believe that the Coccididæ of this country are likely to furnish any products of a useful or commercial character.
[D] A small quantity was submitted to Mr. Skey, Colonial Museum Laboratory, for analysis, no more being available at the time. Mr. Skey considered it as a new substance, probably of the nature of a gum, not resinous; but further examination of larger quantities is necessary.
There is, however, one substance produced by these insects which has an injurious effect upon the plants they grow on. This is a transparent glutinous fluid, apparently analogous to that exuding from Aphides, and which may receive the name of "honeydew," as in that family. In fact, this fluid would seem to be produced by most of the Rhynchota, for the Psyllidæ and Aleurodidæ also excrete it. The quantity issuing from Coccids seems to vary greatly. In some cases—e.g., Lecanium hesperidum, Ctenochiton viridis or perforatus, Fiorinia asteliæ—the insects appear to discharge "honeydew" freely; in others—e.g., Mytilaspis pomorum, Rhizococcus fossor—none, or scarcely any fluid, is excreted. But in no case does it appear that our Coccids[E] form honeydew to the same extent as the Aphides, which are stated to produce sometimes quantities that may be gathered from the leaves or the soil by the pound weight. It is not so much the amount exuding from each insect as the great number of insects on a plant which renders the Coccid honeydew obnoxious: each individual may excrete only a little, but when, as usually happens, there are many hundreds of individuals together, the result, for the reasons given below, becomes important to the tree.
[E] Gossyparia mannipara, an Arabian Coccid, is said to excrete so much that the Arabs "eat it with their bread like honey." Buckton, "Brit. Aphides," Vol. I., p. 42.
There is every reason to believe that the honeydew of Coccididæ is of similar character to that of the Aphididæ, and, according to analyses by Boussingault, of Paris, and Gunning, of Amsterdam (Buckton, "Brit. Aphides," Vol. I., pp. 42, 43), the Aphidian honeydew contains a very large quantity of sugar, and, curiously enough, cane-sugar. Some observers, noticing in its composition also glucose and dextrine, have considered it as of vegetable rather than animal origin; but the weight of evidence appears to make it certainly the product of the Aphides. As the present work is intended rather as a manual for gardeners and tree-growers than as a purely scientific publication, there is no need to enter more fully into the subject here: it may therefore be simply stated that the honeydew of Coccididæ probably contains a large proportion of sugar in various forms.
The mode in which this substance is excreted by the insects differs somewhat from that of the Aphididæ. On the abdomen of Aphis are seen two erect more or less prominent tubes, called "cornicles" or "nectaries," and it is the function of these to excrete the honeydew.[F] No European entomologist has, it is believed, seen or described the organ of honeydew-excretion in the Coccididæ. Some observations by the author of this work in 1886 demonstrate its existence as a cylindrical tube exserted from the anogenital orifice after the manner of a telescope, the furthest-extended tube being the most slender. This organ, extremely difficult of detection when not in use—except in the single genus Cœlostoma—is at intervals pushed out to its full extent, and at its further extremity there appears a minute globule of yellowish, nearly transparent, glutinous fluid, which rapidly expands like a soap-bubble, and then, suddenly breaking, falls in spray on the leaf beneath. In the second stage of the female of Cœlostoma zælandicum this organ may be detected more easily than in any other Coccid; but the act of protrusion of the organ and the formation of the drop of honeydew are apparently by no means frequent, and many long observations may be made without witnessing either.[G] (The organ and the honeydew-drop are shown in [Plate xxii])
[F] The fluid also emerges from the anal orifice; but, seemingly, no mention is made by any observer of any special honeydew-organ protruding from the anus of Aphis.