Fig. 342.—Tremex columba. North America. A, Imago, female: B, pupa, female, ventral aspect: C, larva; a, imperfect legs: D, parasitic larva of Thalessa. (B and D after Riley.)
The Insects of this family are usually of large size and of bright conspicuous colours; these, however, frequently differ greatly in the sexes of the same species, and may be very variable even in one sex. The antennae are filiform and usually elongate; the head is usually contiguous with the thorax, but in one division, Xyphidriides, it is exserted and separated from the thorax by a well-marked neck. The pronotum is attached to the mesonotum, and possesses very little, if any, freedom of movement; it varies in its size, being sometimes conspicuous from above; in the Xyphidriides it is smaller, and in the middle is entirely vertical in its direction. The mesonotum is moderate in size, and its divisions are delimited by broad vague depressions. The prosternum appears to be entirely membranous, but the prosternal plates (pleura) are large, and meet together accurately in the middle, so as to protect the greater part of the under-surface of the neck. The abdomen is cylindrical or somewhat flattened above; it has seven dorsal plates in addition to the spine-bearing terminal segment. The trochanters are double, the outer division being, however, short; the anterior tibia has only one spur; the anal lobe of the posterior wings is large. The "borer" or ovipositor of the female is a remarkable organ; it is held projecting directly backwards from the extremity of the body, and has the appearance of being a powerful sting. The apparatus is much longer than it appears, for it proceeds not from the apex of the body, but from the under-surface far forwards, so that the part exposed is only about one-half of the total length; it consists of a pair of elongate sheaths, which are easily separable though they wrap together, and enclose a slender tube. This tube is rigid and quite straight; though appearing solid, it is really composed of two very perfectly adjusted laminae and a third arched piece or roof. The two lower laminae are called the spiculae; they are serrated or grooved in a peculiar manner near the tip, and although so closely adjusted to the borer or upper piece of the tube as to appear to form one solid whole with it, they are said to be capable of separate motion. In addition to these parts, the termination of the abdomen bears above a shorter piece that projects in a parallel plane, and forms a sort of thick spine above the ventral pieces we have described; this process is very strong, and has in the middle of its under-face in Sirex gigas a membranous cavity, replaced in S. juvencus, according to Westwood, by a pair of minute pilose styles. The Insect, by means of this powerful apparatus, is enabled to deposit her eggs in the solid wood of trees, in which the larva sometimes penetrates to the depth of eight inches.
Sirex gigas is one of the most remarkable of our British Insects, but is little known except to entomologists, being usually rare. On the continent of Europe it is, however, an abundant Insect, especially in the neighbourhood of forests of fir-trees, and is a cause of considerable terror. As the Insect is not capable of inflicting much injury to the person, it is probable that the peculiar ovipositor is believed to be a sting. The eggs are laid—it is said to the number of 100—in the solid wood of fir-trees, but not in perfectly healthy wood; the reason for this, it is thought, being that in a healthy tree the great affluence of sap caused by the burrows and presence of the Insect would be injurious to the latter. The Sirex will, however, attack a perfectly healthy tree immediately after it has been felled. The larva, small at first, enlarges its burrows as itself grows larger, and thus the wood of a tree may be rendered completely useless for trade purposes, although there may be very little outward indication of unsoundness. The larva (Fig. 342, C, larva of Tremex) is a pallid, maggot-like creature, with six projections representing thoracic legs; there are no other legs behind these, but some slight protuberances take their place; the terminal segment is enlarged, and bears a hard spine. There is a difference of opinion as to the duration of the life of the larva, Kollar saying that in seven weeks after the deposition of the egg the maggot is full fed, while others consider that it takes two years to attain this condition; the latter statement is more probably correct, it being the rule that the life of wood-feeding larvae is more than usually prolonged. After becoming full fed, the Insect may still pass a prolonged period in the wood before emerging as a perfect Insect. As a result of this it not infrequently happens that the Insect emerges from wood that has been carried to a distance, and used for buildings or for furniture. A case is recorded in which large numbers of a species of Sirex emerged in a house in this country some years after it was built, to the great terror of the inhabitants. The wood in this case was supposed to have been brought from Canada.
Fabre has studied[[426]] the habits of the larva of Sirex augur, and finds that it forms tortuous galleries in the direction of the longitudinal axis of the tree or limb, and undergoes its metamorphosis in the interior, leaving to the perfect Insect the task of finding its way out; this the creature does, not by retracing its path along the gallery formed by the larva, but by driving a fresh one at right angles to the previous course, thus selecting the shortest way to freedom. By what perception or sense it selects the road to the exterior is quite unknown. Fabre is not able to suggest any sort of perception that might enable the larva to pursue the right course, and considers it must be accomplished by means of some sensibility we do not possess. Fabre's observation is the opposite of what has been recorded in the case of S. gigas, where the larva is said to prepare the way for the exit of the perfect Insect.
Individuals of Sirex are often found in dried and solid wood, encased by metal. When the Insect finds itself so confined, it gnaws its way through the metal, if this be lead, and escapes. The perseverance displayed by the Insect in these circumstances seems to indicate a knowledge of the direction in which liberty is to be found.
About 100 species of Siricidae are known. They form two sub-families:—
1. Siricides: back of head nearly or quite contiguous with the pronotum.
2. Xyphidriides: back of head separated from the pronotum by an elongate neck.
We are reputed to possess in Britain two species of each of these sub-families, but it is doubtful whether more than one Siricid is truly native. Sirex gigas is frequently brought over in timber, and certainly breeds at times freely in Britain. Mr. Leech has recorded the occurrence of the larvae in abundance in fir-trees in the neighbourhood of Dublin. Sirex juvencus is more rarely met with. Xyphidria camelus is doubtless a native, though now apparently rare. It used to occur about old willows, near London, in the New Forest, and, I believe, also in the neighbourhood of Cambridge.
Fam. IV. Tenthredinidae—Sawflies.