The mouth-parts in a generalized form, consist of an upper lip, or _labrum_, which is a part of the head capsule, and a central unpaired _hypopharynx_, two _mandibles_, two _maxillae_ and a lower lip, or _labium_, made up of the fused pair of second maxillae. These parts may be greatly modified, dependent upon whether they are used for biting, sucking, piercing and sucking, or a combination of biting and sucking.
Roughly speaking, insects may be grouped into those which undergo _complete metamorphosis_ and those which have _incomplete metamorphosis_. They are said to undergo complete metamorphosis when the young form, as it leaves the egg, bears no resemblance to the adult. For example, the maggot changes to a quiescent pupa and from this emerges the winged active fly. They undergo incomplete metamorphosis, when the young insect, as it leaves the egg, resembles the adult to a greater or less extent, and after undergoing a certain number of molts becomes s.e.xually mature.
Representatives of several orders have been reported as accidental or faculative parasites of man, but the true parasites are restricted to four orders. These are the Siphunculata; the Hemiptera, the Diptera and the Siphonaptera.
SIPHUNCULATA
The order SIPHUNCULATA was established by Meinert to include the true sucking lice. These are small wingless insects, with reduced mouth-parts, adapted for sucking; thorax apparently a single piece due to indistinct separation of its three segments: the compound eyes reduced to a single ommatidium on each side. The short, powerful legs are terminated by a single long claw. Metamorphosis incomplete.
There has been a great deal of discussion regarding the structure of the mouth-parts, and the relationships of the sucking lice, and the questions cannot yet be regarded as settled. The conflicting views are well represented by Cholodkovsky (1904 and 1905) and by Enderlein (1904).
[Ill.u.s.tration: 64. Pediculus showing the blind sac (_b_) containing the mouth parts (_a_) beneath the alimentary ca.n.a.l (_p_). After Pawlowsky.]
Following Graber, it is generally stated that the mouth-parts consist of a short tube furnished with hooks in front, which const.i.tutes the lower lip, and that within this is a delicate sucking tube derived from the fusion of the labrum and the mandibles. Opposed to this, Cholodkovsky and, more recently, Pawlowsky, (1906), have shown that the piercing apparatus lies in a blind sac under the pharynx and opening into the mouth cavity (fig. 64). It does not form a true tube but a furrow with its open surface uppermost. Eysell has shown that, in addition, there is a pair of chitinous rods which he regards as the h.o.m.ologues of the maxillae.
When the louse feeds, it everts the anterior part of the mouth cavity, with its circle of hooks. The latter serve for anchoring the bug, and the piercing apparatus is then pushed out.
[Ill.u.s.tration: 65. Pediculus huma.n.u.s, ventral aspect of male. (10)]
Most writers have cla.s.sed the sucking lice as a sub-order of the Hemiptera, but the more recent anatomical and developmental studies render this grouping untenable. An important fact, bearing on the question, is that, as shown by Gross, (1905), the structure of the ovaries is radically different from that of the Hemiptera.
Lice infestation and its effects are known medically as _pediculosis_.
Though their continued presence is the result of the grossest neglect and filthiness, the original infestation may be innocently obtained and by people of the most careful habits.
Three species commonly attack man. Strangely enough, there are very few accurate data regarding their life history.
_Pediculus huma.n.u.s_ (fig. 65), the head louse, is the most widely distributed. It is usually referred to in medical literature as _Pediculus capitis_, but the Linnean specific name has priority. In color it is of a pale gray, blackish on the margins. It is claimed by some authors that the color varies according to the color of the skin of the host. The abdomen is composed of seven distinct segments, bearing spiracles laterally. There is considerable variation in size. The males average 1.8 mm. and the females 2.7 mm. in length.
The eggs, fifty to sixty in number, stick firmly to the hairs of the host and are known as nits. They are large and conspicuous, especially on dark hair and are provided with an operculum, or cap, at the free end, where the nymphs emerge. They hatch in about six days and about the eighteenth day the young lice are s.e.xually mature.
[Ill.u.s.tration: 66. Pediculosis of the head. The ill.u.s.tration shows the characteristic indications of the presence of lice, viz: the occipital eczema gluing the hairs together, the swollen cervical glands, and the porrigo, or eruption of contagious pustules upon the neck. After Fox.]
The head lice live by preference on the scalp of their host but occasionally they are found on the eyelashes and beard, or in the pubic region. They may also occur elsewhere on the body. The penetration of the rostrum into the skin and the discharge of an irritating saliva produce a severe itching, accompanied by the formation of an eczema-like eruption (fig. 66). When the infestation is severe, the discharge from the pustules mats down the hair, and scabs are formed, under which the insects swarm. "If allowed to run, a regular carapace may form, called _trichoma_, and the head exudes a ftid odor. Various low plants may grow in the trichoma, the whole being known as _plica palonica_."--Stiles.
Sources of infestation are various. School children may obtain the lice from seatmates, by wearing the hats or caps of infested mates, or by the use, in common, of brushes and combs. They may be obtained from infested beds or sleeper berths. Stiles reports an instance in which a large number of girls in a fashionable boarding school developed lousiness a short time after traveling in a sleeping car.
Treatment is simple, for the parasites may readily be controlled by cleanliness and washing the head with a two per cent solution of carbolic acid or even kerosene. The latter is better used mixed with equal parts of olive oil, to avoid irritation. The treatment should be applied at night and followed the next morning by a shampoo with soap and warm water. It is necessary to repeat the operation in a few days.
Xylol, used pure, or with the addition of five per cent of vaseline, is also very efficacious. Of course, the patient must be cautioned to stay away from a lighted lamp or fire while using either the kerosene or xylol. While these treatments will kill the eggs or nits, they will not remove them from the hairs. Pusey recommends repeated washings with vinegar or 25 per cent of acetic acid in water, for the purpose of loosening and removing the nits.
Treatment of severe infestations in females is often troublesome on account of long hair. For such cases the following method recommended by Whitfield (1912) is especially applicable:
The patient is laid on her back on the bed with her head over the edge, and beneath the head is placed a basin on a chair so that the hair lies in the basin. A solution of 1 in 40 carbolic acid is then poured over the hair into the basin and sluiced backwards and forwards until the whole of the hair is thoroughly soaked with it. It is especially necessary that care should be taken to secure thorough saturation of the hair over the ears and at the nape of the neck, since these parts are not only the sites of predilection of the parasites but they are apt to escape the solution. This sluicing is carried out for ten minutes by the clock. At the end of ten minutes the hair is lifted from the basin and allowed to drain, but is not dried or even thoroughly wrung out. The whole head is then swathed with a thick towel or better, a large piece of common house flannel, which is fastened up to form a sort of turban, and is allowed to remain thus for an hour. It can then be washed or simply allowed to dry, as the carbolic quickly disperses. At the end of this period every pediculus and what is better, every ovum is dead and no relapse will occur unless there is exposure to fresh contagion.
Whitfield states that there seem to be no disadvantages in this method, which he has used for years. He has never seen carboluria result from it, but would advise first cutting the hair of children under five years of age.
_Pediculus corporis_ (= _P. vestimenti_) the body louse, is larger than the preceding species, the female measuring 3.3 mm., and the male 3 mm.
in length. The color is a dirty white, or grayish. _P. corporis_ has been regarded by some authorities as merely a variety of _P. huma.n.u.s_ but Piaget maintains there are good characters separating the two species.
The body louse lives in the folds and seams of the clothing of its host, pa.s.sing to the skin only when it wishes to feed. Brumpt states that he has found enormous numbers of them in the collars of gla.s.s-ware or grains worn by certain naked tribes in Africa.
Exact data regarding the life-history of this species have been supplied, in part, by the work of Warburton (1910), cited by Nuttall. He found that _Pediculus corporis_ lives longer than _P. huma.n.u.s_ under adverse conditions. This is doubtless due to its living habitually on the clothing, whereas _huma.n.u.s_ lives upon the head, where it has more frequent opportunities of feeding. He reared a single female upon his own person, keeping the louse enclosed in a cotton-plugged tube with a particle of cloth to which it could cling. The tube was kept next to his body, thus simulating the natural conditions of warmth and moisture under which the lice thrive. The specimen was fed twice daily, while it clung to the cloth upon which it rested. Under these conditions she lived for one month. Copulation commenced five days after the female had hatched and was repeated a number of times, s.e.xual union lasting for hours. The female laid one hundred and twenty-four eggs within twenty-five days.
The eggs hatched after eight days, under favorable conditions, such as those under which the female was kept. They did not hatch in the cold.
Eggs kept near the person during the day and hung in clothing by the bedside at night, during the winter, in a cold room, did not hatch until the thirty-fifth day. When the nymphs emerge from the eggs, they feed at once, if given a chance to do so. They are p.r.o.ne to scatter about the person and abandon the fragment of cloth to which the adult clings.
The adult stage is reached on the eleventh day, after three molts, about four days apart. Adults enter into copulation about the fifth day and as the eggs require eight days for development, the total cycle, under favorable conditions, is about twenty-four days. Warburton"s data differ considerably from those commonly quoted and serve to emphasize the necessity for detailed studies of some of the commonest of parasitic insects.
Body lice are voracious feeders, producing by their bites and the irritating saliva which they inject, rosy elevations and papules which become covered with a brownish crust. The intense itching provokes scratching, and characteristic white scars (fig. 67) surrounded by brownish pigment (fig. 68) are formed. The skin may become thickened and take on a bronze tinge. This melanoderma is especially marked in the region between the shoulders but it may become generalized, a prominent characteristic of "vagabond"s disease." According to Dubre and Beille, this melanoderma is due to a toxic substance secreted by the lice, which indirectly provokes the formation of pigment.
[Ill.u.s.tration: 67. Pediculosis in man caused by the body louse. After Morrow.]
Control measures, in the case of the body louse, consist in boiling or steaming the clothes or in some cases, sterilizing by dry heat. The dermat.i.tis may be relieved by the use of zinc-oxide ointment, to which Pusey recommends that there be added, on account of their parasiticidal properties, sulphur and balsam of Peru, equal parts, 15 to 30 grains to the ounce.
_Phthirius pubis_ (= _P. inguinalis_), the pubic louse, or so-called "crab louse," differs greatly from the preceding in appearance. It is characterized by its relatively short head which fits into a broad depression in the thorax. The latter is broad and flat and merges into the abdomen. The first pair of legs is slender and terminated by a straight claw. The second and third pairs of legs are thicker and are provided with powerful claws fitted for clinging to hairs. The females (fig. 69) measure 1.5 to 2 mm. in length by 1.5 mm. in breadth. The male averages a little over half as large. The eggs, or nits, are fixed at the base of the hairs. Only a few, ten to fifteen are deposited by a single female, and they hatch in about a week"s time. The young lice mature in two weeks.
The pubic louse usually infests the hairs of the pubis and the perineal region. It may pa.s.s to the arm pits or even to the beard or moustache.
Rarely, it occurs on the eyelids, and it has even been found, in a very few instances, occurring in all stages, on the scalp. Infestation may be contracted from beds or even from badly infested persons in a crowd. We have seen several cases which undoubtedly were due to the use of public water closets. It produces papular eruption and an intense pruritis.
When abundant, there occurs a grayish discoloration of the skin which Duguet has shown is due to a poisonous saliva injected by the louse, as is the melanoderma caused by the body louse.
[Ill.u.s.tration: 68. Melanoderma caused by the body louse. From Portfolio of Dermochromes, by permission of Rebman & Co., New York, Publishers.]
The pubic louse may be exterminated by the measures recommended for the head louse, or by the use of officinal mercurial ointment.
[Ill.u.s.tration: 69. Phthirius pubis. Ventral aspect of female. (12).]
HEMIPTERA
Several species of HEMIPTERA-HETEROPTERA are habitual parasites of man, and others occur as occasional or accidental parasites. Of all these, the most important and widespread are the bed-bugs, belonging to the genus _Cimex_ (= _Acanthia_).
THE BED-BUGS--The bed-bugs are characterized by a much flattened oval body, with the short, broad head unconstricted behind, and fitting into the strongly excavated anterior margin of the thorax. The compound eyes are prominent, simple eyes lacking. Antennae four-jointed, the first segment short, the second long and thick, and the third and fourth slender. The tarsi are short and three segmented.
It is often a.s.sumed in the literature of the subject that there is but a single species of _Cimex_ attacking man, but several such species are to be recognized. These are distinguishable by the characters given in Chapter XII. We shall consider especially _Cimex lectularius_, the most common and widespread species.
_Cimex lectularius_ (= _Acanthia lectularia_, _Clinocoris lectularius_), is one of the most cosmopolitan of human parasites but, like the lice, it has been comparatively little studied until recent years, when the possibility that it may be concerned with the transmission of various diseases has awakened interest in the details of its life-history and habits.
[Ill.u.s.tration: 70. Cimex lectularius adult and eggs. Photograph by M. V.
S.]
The adult insect (fig. 70) is 4-5 mm. long by 3 mm. broad, reddish brown in color, with the beak and body appendages lighter in color. The short, broad and somewhat rectangular head has no neck-like constriction but fits into the broadly semilunar prothorax. The four segmented labium or proboscis encloses the lancet-like maxillae and mandibles. The distal of the four antennal segments is slightly club-shaped. The prothorax is characteristic of the species, being deeply incised anteriorly and with its thin lateral margins somewhat turned up. The mesothorax is triangular, with the apex posteriorly, and bears the greatly atrophied first pair of wings. There is no trace of the metathoracic pair. The greatly flattened abdomen has eight visible segments, though in reality the first is greatly reduced and has been disregarded by most writers.
The body is densely covered with short bristles and hairs, the former being peculiarly saber-shaped structures sharply toothed at the apex and along the convex side (fig. 159_b_).
The peculiar disagreeable odor of the adult bed-bug is due to the secretion of the stink glands which lie on the inner surface of the mesosternum and open by a pair of orifices in front of the metac.o.xae, near the middle line. In the nymphs, the thoracic glands are not developed but in the abdomen there are to be found three unpaired dorsal stink glands, which persist until the fifth molt, when they become atrophied and replaced by the thoracic glands. The nymphal glands occupy the median dorsal portion of the abdomen, opening by paired pores at the anterior margin of the fourth, fifth and sixth segments. The secretion is a clear, oily, volatile fluid, strongly acid in reaction. Similar glands are to be found in most of the Hemiptera-Heteroptera and their secretion is doubtless protective, through being disagreeable to the birds. In the bed-bug, as Marlatt points out, "it is probably an ill.u.s.tration of a very common phenomenon among animals, i.e., the persistence of a characteristic which is no longer of any special value to the possessor." In fact, its possession is a distinct disadvantage to the bed-bug, as the odor frequently reveals the presence of the bugs, before they are seen.
The eggs of the bed-bug (fig. 70) are pearly white, oval in outline, about a millimeter long, and possess a small operculum or cap at one end, which is pushed off when the young hatches. They are laid intermittently, for a long period, in cracks and crevices of beds and furniture, under seams of mattresses, under loose wall paper, and similar places of concealment of the adult bugs. Girault (1905) observed a well-fed female deposit one hundred and eleven eggs during the sixty-one days that she was kept in captivity. She had apparently deposited some of her eggs before being captured.
The eggs hatch in six to ten days, the newly emerged nymphs being about 1.5 mm. in length and of a pale yellowish white color. They grow slowly, molting five times. At the last molt the mesathoracic wing pads appear, characteristic of the adult. The total length of the nymphal stage varies greatly, depending upon conditions of food supply, temperature and possibly other factors. Marlatt (1907) found under most favorable conditions a period averaging eight days between molting which, added to an equal egg period, gave a total of about seven weeks from egg to adult insect. Girault (1912) found the postembryonic period as low as twenty-nine days and as high as seventy days under apparently similar and normal conditions of food supply. Under optimum and normal conditions of food supply, beginning August 27, the average nymphal life was 69.9 days; average number of meals 8.75 and the molts 5. Under conditions allowing about half the normal food supply the average nymphal life was from 116.9 to 139 days. Nymphs starved from birth lived up to 42 days. We have kept unfed nymphs, of the first stage, alive in a bottle for 75 days. The interesting fact was brought out that under these conditions of minimum food supply there were sometimes six molts instead of the normal number.