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case of cockchafers (Melolontha) ; the sixteen-footed larvæ of butterflies and moths (those of spanners have only ten, and a few mining-moths have none) are called caterpillars ; the eighteen- to twenty-two-footed larvæ of sawflies with their taillike extremities are named tailed caterpillars; while the larvæ of flies (Diptera), which have neither feet nor any complex structure of the head, are called maggots. The larvæ of many beetles have three pairs of legs on the first three (thoracic) segments after the head ; the caterpillars of moths and butterflies have these also, and in addition from two to five pairs of prolegs or clasping feet attached to the abdominal segments, the last pair of which (claspers or anal prolegs) are on the terminal segment. As it gradually grows in size, the larva moults its skin several times before pupation. The pupa sometimes lies unprotected on the ground under moss and dead foliage, or in fissures or under bark-scales, and at other times it•is enclosed within a woven cocoon (often of large size in some of the Bombycidæ), while with flies (Diptera) the last larval skin forms a protective covering. The longest stage of development is that in which the insect hibernates ; and this is very often the larval stage, although many of the Coleoptera hibernate as beetles. The egg and pupal stages usually last only from two to four weeks, except with insects which hibernate thus.
As soon as the mature insects appear, they usually pair at once and reproduce themselves. The male generally dies soon after pairing,—except in the case of beetles, which often hibernate, and of bees, which live for four or five years.
The Generation of any insect, or the complete cycle from egg to egg, varies greatly. It is multiple in plant-lice and ichneumons, which produce several generations within the year ; double in some bark-beetles and sawflies, which produce two generations in each year ; single, simple, or annual in the case of most butterflies and moths, which yearly produce one generation ; biennial or two-yearly in wood-wasps, the Pine resin-gall tortrix, and many longicorn beetles; and plurennial in the cockchafer, which takes at least three and usually four years to complete its generation. Occasionally two generations take place in three years (as in Bostrichus bidens), but this is exceptional.
Metabolic insects feed merely as larvæ and mature insects, and in some exceptional cases (e.g., the Pine-weevil) only do damage as imagines ; but with ametabolic insects the nymph also feeds. In both groups the larvæ are often very
destructive, and many
of them daily devour many times their own weight of foliage. Insects injurious to Woodlands.- Insects might be classed with reference to the trees on which they mainly feed (Pine insects, &c.), but this would be misleading, because many are not merely monophagous but also polyphagous, and even pantophagous, devouring everything when occurring in vast swarms. Or they might be classed with reference to the kind of damage they do, which may be either physiological (interfering with the imbibition, transpiration, or assimilation of food and nourishment, as in bark-beetles and moths) or technical (interfering with the value of timber for technical purposes, as in wood-wasps, longicorns, &c.) ; but this would also be inconvenient, as a great deal depends on species and age of tree, time of attacks, surrounding local circumstances, &c. Or again, they might be classified with reference to the extent of the damage usually done ; but this would be very unsafe, because some of the most destructive kinds do little damage if kept in check, yet become exceedingly destructive if circumstances favour their development in enormous numbers, when they may devastate extensive woods.1
1 A rough classification into (1) slightly, (2) noticeably, and (3) very injurious is too vague and indefinite. Experience has shown that the large and the small Pine-weevils, the cockchafer, the Lareh mining-moth, and the Pine sawfly are, or may at any time become, very injurious, although under ordinary circumstances they can be kept in check and prevented from doing much damage. The extent of injury that may be done depends entirely on circumstances. Whenever injurious insects become apparent, the forester’s motto should be Principiis obsta—“take time by the forelock”—and try to exterAny attempt to group them with reference to the age of the crops generally attacked (seedling-growth, thickets, pole-woods, and highwoods) is also unsatisfactory, as many insects are dangerous at all stages of tree-growth. But it may be remarked that most weevils, some leaf-rollers, and the cockchafer grubs usually attack seedling growth and young thickets, and that pole-woods and older crops are most exposed to attacks from moths, and then of bark, and cambial-beetles, when physiological disturbances arise and the trees grow sickly. The caterpillars of the Pine owlet-moth and the Pine span-worm always attack pole-woods before migrating to older crops on their numbers increasing largely. In Germany a biological method of classification is often preferred, the noxious insects being classified with reference to the part of the tree usually injured. Such classification distinguishes
1. Root-destroyers, such as the mole-cricket and the cockchafer grub.
2. Wood-borers, comprising the larvæ of wood-wasps, cervicorn beetles, certain barkbeetles, and the caterpillars of goat-moths.
3. Bark-beetles, including most of the Scolytidæ and several weevils, which often, both as beetle and larva, either destroy the cambium and the sapwood (e.g., most Bostrichini and several Curculionidæ), or else hollow out the pith in young shoots (e.g., some Hylesinini and Tortricida).
4. Bud- and Leaf-destroyers, including several weevils, the caterpillars of most moths and sawflies, also leaf-beetles and cockchafers.
5. Producers of Deformities and Malformations on foliage, shoots, and fruits by gall-wasps, gall-midges, plant-lice, which are sometimes of much consequence (e.g., as in the case of the Larch-aphis).
For the forester, perhaps the simplest and most practical classification of insects is the old-fashioned one according to the natural orders formulated by Burmeister, and based on the morphology of the mature insect :I. Insects with complete metamorphosis (showing distinctly the four different stages of Ovum, Larva, Pupa, and Imago). 1. Imago with biting mouth-parts. (1) Imago with two wing pairs, of which the fore-pair are merely horny
shields or wing-cases (elytra), and only the inembranous hind-pair are used in flying
Coleoptera. (2) Imago with two pairs of membranous wings with many veins or
Neuroptera. 2. Imago with biting mouth-parts, or partly biting and partly sucking mouth. parts, and two pairs of membranous wings with comparatively few veins
Hymenoptera. 3. Imago with sucking mouth-parts. (1) Imago with two pairs of membranous wings wholly or partially covered with scales
Lepidoptera. (2) Imago with the pair of fore-wings membranous and well-developed, but
the hind pair only rudimentary and aborted into small stalked knobs (poisers)
minate the pest before it develops into a scourge, and especially in coniferous forests. Monophagous species on multiplying rapidly become polyphagous and finally pantophagous, devouring the foliage of all kinds of trees.
It would be hard to overrate the importance of discovering any abnormal increase in the usual number of injurious insects, and of taking early steps to exterminate them. Pests that might be exterminated during the first spring at a slight cost may entail a much greater outlay in the following and subsequent years, and great loss may at the same time be involved by having to fell and reproduce immature crops badly damaged. In our comparatively small woodlands area, there is no possibility of vast calamities such as have sometimes necessitated the clearance and replantation of millions of trees in Central Europe. But such devastations teach lessons worth learning, namely—(1) that mixed woods are least attacked by insects, and (2) that the best way of preventing attacks is to keep the woods clean, well-thinned, and well-looked-after at all stages of their growth.
II. Insects with incomplete metamorphosis (in which there are no distinct larval and pupal stages). 1. With biting mouth-parts
Orthoptera. 2. Imago with sucking mouth-parts.
Hemiptera. But as regards their relative importance, it is perhaps best to consider them. in the following order : (1) Coleoptera, (2) Lepidoptera, (3) Hymenoptera, and (4) Diptera, then (5) Hemiptera and (6) Orthoptera. The order Neuroptera contains no insects injurious to woodlands.
The following table shows the chief insects that have as yet proved more or less injurious to woodlands and nurseries in Britain, those which are sometimes very destructive being marked with an asterisk, thus * :
B. Proboscid 1. *Hylobius abietis Beetle
Larva ; lapathi
beetle 6. Strophosoius coryli Larva ;
Scots Pine, Bark of 89
1. Cheimatobia bru- Caterpillar Fruit-trees; Foliage
114 115 116 116
V. Half-Winged A. Plant-lice Insects
B. Scale Insects
1. Chermes abietis During all Spruce
stages 2. * Chermes laricis During all Larch
stages 3. Chermes coccineus During all Spruce
stages 4. Tetraneura ulmi During all Elm
stages 5. Schizoneura lanu- During all Elm ginosa
stages 6. Chermes fraxini
During all Ash
During all Beech
1. Cryptococcus fagi
Chermes fagi) 2. Apterococcus
fraxini 3. Coccus salicis
During all Ash Bark
stages During all Ash, Willow Bark
During all Nursery