Chapter XXV: Appendix: “Rhizopods in Poliomyelitis acuta.” (19)

Male 0·5 mm. in length by 0·25 mm. in breadth; female 0·53 mm. in length by 0·28 mm. in breadth; the males have two suckers on the tarsi of the fourth pair of legs. Penis straight, colour whitish or reddish.

*Tyroglyphus longior*, Gervais, 1844.

White or yellowish, with two black spots on the abdomen. Male 0·55 mm. in length, 0·28 mm. in breadth; penis bent. Female 0·61 mm. in length and 0·28 mm. in breadth.

_T. siro_ and _T. longior_ live on dry cheese, in flour, on dried fruits, etc., and have been occasionally observed in the stools, urine, or pus of human beings, and also on their skin. The so-called vanillismus is to be attributed to these species.

[_T. siro_ and _T. farinæ_ of Schrank (non Geer) are the same. They are described under other names, such as _Acarus lactis_, Linn.; _A. favorum_, Herm., etc.; _A. lactis_ in milk, _farinæ_ in flour, and _siro_ in cheese; and as _A. dysenteriæ_, Linnæus (“Syst. Nat.,” ed. 12, pp. 1024–1767).]

It is to these species that a case of dysentery was referred. Rolander, who studied under Linnæus, was attacked by what was called dysentery. The complaint soon gave way to treatment, but eight days after it returned, soon disappeared, but again came a third time. All the time Rolander had been living like the other inmates of the house, who all escaped. Linnæus, aware that Bartholemy had attributed dysentery to insects which he said he had seen, advised his student to examine his stool. The result was that innumerable mites were found to be present. Their presence was easily accounted for by the fact that they were found in numbers in a cup made of juniper wood from which the student alone drank of a night, and they were found to be of the same species. What this species is we do not know. Linnæus called it _Acarus dysenteriæ_, but it was the same as his _Acarus siro_. No records have occurred since. It cannot be, as Latreille supposed, the cheese mite, for they have been eaten by millions since, and it is strange no such case has occurred again.

[*Tyroglyphus minor* var. *Castellani*, Hirst,

causes the copra itch in persons employed in the copra mills in Ceylon. The skin of the hands, arms, legs and even body becomes covered with pruriginous papules, papulo-pustules and pustules near the head. The eruption begins as a rule on the hands. The mites live in the copra dust. They produce dermatitis. Castellani produced the disease experimentally by rubbing copra dust containing mites on the skin of healthy people. Beta-naphthol ointment (5 to 10 per cent.) proved useful in treatment (_Journ. Trop. Med. and Hyg._, December 16, 1912, Castellani and Hirst).--F. V. T.]

Genus. *Glyciphagus*, Hering, 1838.

*Glyciphagus prunorum*, Her., and *G. domesticus*, de Geer.

The Glyciphagi are differentiated from the Tyroglyphi in that the chitinous hairs on the body are fringed or feathered, and that they lack a furrow dividing the cephalothorax from the abdomen. They live under similar conditions to the Tyroglyphi and are occasionally found on man or in fæces.

[Sugar merchants and grocers are frequently troubled by swarms of _G. domesticus_, which leave the stores when being handled, and especially shopmen, who handle sugar kept in small stores for some time. These are the Acari that cause that irritating temporary affection known as “grocer’s itch.”--F. V. T.]

*Glyciphagus cursor*, Gervais.

Under this name Signor Moriggia figures a horny excrescence of great length growing from a woman’s hand, and containing in its cavities quantities of Acarus. This species is really _G. domesticus_, de Geer. _G. domesticus_ has also been described by Gervais (_Ann. Sci. Nat._, 1841, ser. 2, xv, p. 8) as _G. hippopodes_.

*Glyciphagus buski*, Murray.[350]

[350] Cooper and Busk’s _Micros. Journ._, 1842, and “Economic Entomology,” Murray, p. 280.

[This is a mite found by Busk and named after him by Murray. It was taken from beneath the cuticle of the sole of the foot of a negro in the Seamen’s Hospital Ship on the Thames in 1841, in large sores of a peculiar character confined to the soles of the feet. It appeared that the disease was caused by its burrowing beneath the thick cuticle. The disease was attributed to the wearing of a pair of shoes which had been lent to another negro whose feet had been similarly affected for nearly a year. The negro to whom the shoes were lent came from Sierra Leone. Mr. Busk stated that some water brought by Dr. Stranger from the River Sinoe, on the coast of Africa, contained one nearly perfect specimen, and fragments of others very similar to if not identical with this Acarus. Mr. Busk adds that he had been informed by Staff-Assistant Surgeon P. D. Murray that at Sierra Leone there is a native pustular disease called craw-craw--a species of itch breaking into open sores.

[From Busk’s original figure I see no reason to doubt that this is a Glyciphagus.--F. V. T.]

Genus. *Rhizoglyphus*, Claparède, 1869.

*Rhizoglyphus parasiticus*, Dalgetty, 1901.

The _Rhizoglyphii_ are to be recognized by their short legs, which
are beset with spines, and by the tarsi, which terminate in a claw.
They live on plants, roots and bulbs, especially the bulbs of lilies.

This species has been observed on the feet of Indian coolies working in the tea plantations; they produce a skin disease which always commences with blebs between the toes, and which almost always extends to the malleoli, but not beyond. The Acari have an elliptical body, which is grey, but varies from greenish-yellow to greenish-brown when the stomach is full. Eyes are absent. The legs are composed of five segments and terminate with a claw. The males measure 0·18 mm. in length by 0·08 mm. in breadth, and possess genital and anal pores; the females measure 0·2 mm. in length by 0·09 mm. in breadth.[351] [This is also known as coolie itch and is common in Indian tea plantations.--F. V. T.]

[351] Dalgetty, A. B., “Water-itch; or Sore Feet of Coolies,” _Journ. Trop. Med._, 1901, iv, p. 73.

Genus. *Histiogaster*, Berlese, 1883.

*Histiogaster* (*entomophagus?*) *spermaticus*, Trouessart, 1900.

The genus Histiogaster, which also approaches the _Tyroglyphinæ_, is
characterized by the circumstance that the males possess suctorial
pores used in copulation, as well as leaf-shaped appendages at the
posterior end of the body. They feed on vegetables, especially on
small fungi.

This species has been described by Trouessart,[352] who found numerous specimens, some adult, others in the developmental stage (larvæ, nymphs), and ova, in the fluid removed by puncture from a cyst of the right testis. The males measure 0·25 mm., the females 0·32 mm., and the larvæ 0·1 mm. in length. The author is of opinion that the animal--perhaps a fertilized female--was introduced by a catheter, and, as a matter of fact, it was afterwards found that the patient had once had the catheter passed in India while suffering from pernicious fever.

[352] Trouessart, E. R., _Compt. rend. Soc. Biol._, Paris, 1900, lii, pp. 742–744, 893, 894; _Arch. de Par._, 1902, v, pp. 449–459.

It would here rather appear to be the case of a facultative parasitism of an otherwise free-living species. _Histiogaster entomophagus_, Laboulbène, is found occasionally in collections of insects feeding on larger species containing much fat; the species also occurs on dry cantharides; it appears to belong to the region of South Europe, where, however, it is widely spread.

[Entomophagus occurs all over Europe and in America. It has been described under the following names: _Acarus malus_, Shimer, 1868 (_Trans. Illinois Hort. Soc._); _Dermaleichus mali_, Riley, 1873 (_Rep. Ins. Missouri_, v, p. 87); _Tyroglyphus mali_, Murray, 1877 (“Eco. Ent. Apt.,” p. 275); _T. corticalis_, Michael, 1885 (_Trans. Roy. Micros. Soc._, ser. 2, v, 3, p. 27, figs. 1 to 14); _Histiogaster corticalis_, Canestrini, 1888 (_Prosp. Acarof._, iii, p. 397); _H. aleurophagus_, Sicherin, 1894, Canestrini, _Prosp. Acarof._, vi, p. 815. Trouessart’s species is evidently distinct.--F. V. T.]

Genus. *Cheyletus.*

*Cheyletus mericourti*, Lab.

_Acaropsis mericourti_, Moq. Tand.

[This mite has been described from three specimens found in pus which flowed from an abscess in the ear of a naval officer, produced by inflammation of the auditory passage. Where the mites came from we do not know, as they were found near the Bank of Newfoundland. This genus of Acari has enormous mandibles and a peculiar tracheal system; two ungues and appendages to the tarsi.--F. V. T.]

Family. *Sarcoptidæ* (Itch Mites).

Small mites without eyes and tracheæ, and with delicate, transversely
striated cuticle. The mouth parts form a cone, over which the
shield-shaped upper lip protrudes; the cheliceræ are chelate; the
pedipalpi (or maxillary palpi) have three joints; the legs are short
and compact, and composed of five segments; the terminal joints have
pedunculated suckers (ambulacra) or a long bristle. The larvæ are
six-legged. They live on or under the skin of birds and mammals, on
which they produce the skin disease known as scabies, or itch.

[The _Sarcoptidæ_ attack the hairs, feathers or epidermis of birds,
animals and man, living as permanent parasites. The punctures they
produce are followed by the formation of more or less thick crusts
or scabs, beneath which the mites live and breed (so called scab,
mange and itch). Most are oviparous, some ovoviviparous. The eggs are
minute, ovoid, with a thin semi-transparent shell. They incubate in
a few days, varying from two to ten or eleven, as a rule. Generally
sarcoptic diseases lie dormant in winter and revive in spring and
summer in man; but in animals with long wool, such as sheep, they are
most active during winter, although revival of active reproduction
takes place in spring.

[Speaking generally, for the _Sarcoptidæ_ there are three distinct
stages in the development of the male, four in the female, as
follows:--

[(1) The larva. In this stage only three pairs of legs occur.

[(2) The nymph, in which a fourth pair of legs appear, and which thus
approaches the adult; but so far no sexual organs occur. Nymphs are
of two sizes--the smaller being future males, the larger females.

[(3) The next stage in the female is the age of _puberty_, the female
now being provided with a vulvo-anal slit; this so-called _pubescent
female_ is fertilized by the male. The male then dies. But the female
again casts her skin and enters another stage--

[(4) The ovigerous female--the egg-laying female--which has
differently modified legs.

[The rate at which these Acari breed is very great. Gerlach has
found that roughly, in each Sarcopt gallery, a female produces
fifteen individuals--ten females and five males--and that the
progeny reproduce again in fifteen days. The table given below thus
shows that one pair may produce the enormous number of 1,500,000
descendants in three months:--

First generation after 15 days 10 females 5 males
Second " " 30 " 100 " 50 "
Third " " 45 " 1,000 " 500 "
Fourth " " 60 " 10,000 " 5,000 "
Fifth " " 75 " 100,000 " 50,000 "
Sixth " " 90 " 1,000,000 " 500,000 "

= _1,500,000 individuals._

[These _Acarinæ_ are divided into three distinct sub-families, namely
the _Cytolichinæ_, _Sarcoptinæ_, _Canestriniinæ_.

[The _Sarcoptinæ_ alone interest us here, and of the nine genera the
three following are the most important:--

[(1) Sarcoptes, Latreille; Eusarcoptes.

[(2) Psoroptes, Gerv.; Dermatodectes, Gerlach; Dermatocoptes,
Fürstenberg.

[(3) Chorioptes, Gerv.; Symbiotes, Gerlach; Dermatophagus, Fürst.;
Sarco-dermatocedes, Del.

[The following are the main characters of these three genera:--

[_Sarcoptes_--round or slightly oval; the two posterior pairs of legs
being nearly or quite concealed beneath the body; the tarsi end in
simple long pedicles, with ambulatory suckers.

[_Psoroptes_--oval; the legs are all visible outside the margin of
the body; the ambulatory suckers are carried on long triangulated
stalks; the male has copulatory suckers and abdominal prolongations.

[_Chorioptes_--oval; legs long, thick, all visible; ambulatory
suckers very wide, carried at the end of simple, short pedicles.

[Sarcoptes make channels or furrows beneath the epidermis, and in
these the female lays her eggs. This form of acariasis is thus
difficult to cure. It is the cause of human itch (_vide Sarcoptes
scabiei_).

[Psoroptes do not make sub-epidermic galleries; they live and breed
in colonies beneath crusts or scabs formed by the changes they
produce in their host’s skin. Sheep scab is a common type of disease
produced by Psoroptes. This genus is of little importance as a
parasite to man.

[Chorioptes live as Psoroptes; they also do not affect man.
Otodectes, Can., affecting cats and dogs, and others occur, but
do not affect man as far as we know at present (“Demodicidae und
Sarcoptidae,” von Professor G. Canestrini und P. Kramer, _Das
Tierreich_, 1899).--F. V. T.]

Sub-family. *Sarcoptinæ.*

Genus. *Sarcoptes*, Latreille.

*Sarcoptes scabiei*, de Geer, 1778.

Syn.: _Acarus scabiei_, de Geer, 1778; _A. psoricus_, Pallas, 1760;
_A. siro_, L., 1778; _Sarcoptes exulcerans_,? Linn., 1758, Nitsch,
1818; _S. hominis_, Raspail, 1834, and Hering, 1838; _S. galei_,
Owen, 1853; _S. communis_, Delaf. et Bourg., 1862; _S. scabiei_ var.
_hominis_, Mégnin, 1880.

The body is oval or nearly circular and whitish in colour, with transverse rows of striæ partly interrupted on the back. There are transverse rows of small bristles on the dorsal surface, and groups of trichomæ on the front, sides and back. There are chitinous hairs at the base of the legs; the two first pairs are provided with pedunculated ambulacra in both sexes, the two posterior pairs terminate each with a long bristle in the female; in the male the third pair of legs terminate in a bristle, the fourth pair with a pedunculated ambulacrum. The anus is situated at the posterior border of the dorsal surface.

At one time numerous species were differentiated, according to the
form of the Acarus, the number, position and size of the hairs and
spines, even according to the hosts, etc. All these characteristics,
however, fluctuate so considerably that absolute differentiation is
impossible; the supposed species may be regarded in the same light as
Mégnin did, as varieties. It is also hardly possible to distinguish
the mite of human scabies (_S. hominis_) from that of a number of
domestic animals (_S. squamiferus)_. It is best, therefore, to accept
one single species (_S. scabiei_), which may give rise to different
races or castes by living in the skin of man and mammals, but can
pass from one host to the other.

[Canestrini and Kramer, in their monograph of the _Sarcoptidæ_, enumerate eighteen distinct species of this genus, from the dog, goat, camel, horse, ferret, lion, wolf, sheep, pig, etc., and two species parasites of man (_scabiei_ and _scabiei-crustosæ_). There is no doubt that they are distinct species.--F. V. T.]

The _S. scabiei_ of man (_S. scabiei_ var. _hominis_) (length of male 0·2 to 0·3 mm., and breadth 0·145 to 0·190 mm.; length of female 0·33 to 0·45 mm., and breadth 0·25 to 0·35 mm.) lives in the tunnels that it excavates in the epidermis, and attacks by preference places with thin skin, such as between the fingers, in the bend of the elbows and knees, in the inguinal region, on the penis, on the mammæ, but may also affect other parts. The tunnels, which vary from a few millimetres to a centimetre and more long, do not run straight, but are somewhat tortuous; the female is found at the terminal end. The tunnels contain the excrement and oval eggs (0·14 mm. in length) of the parasite; the males are rarely met with, as they die off after copulation; the females die after depositing their eggs. The six-legged larvæ hatch out after four to eight days, and after about a fortnight, during which time they change their skins three times and undergo metamorphosis, they begin themselves to burrow. Transmission from person to person rarely is effected through linen, but by direct contact (as in coitus); transmission can be artificially effected on horses, dogs and monkeys, but not on cats.

The smaller _S. scabiei-crustosæ_, Fürstenberg, is the cause of the itch that occurs chiefly in Norway; it is not certain whether this is a distinct species of itch mite.

[This is quite a distinct species, which is recorded from Germany and France. Mégnin (_Parasitology_, 1880, p. 165) described this as _S. scabiei_ var. _lupi_. The female is 140 µ long, 340 µ broad; the male is 170 µ long by 150 µ broad. In _Science_ (March 3, 1893, p. 125) is recorded that at the Indiana Academy of Science Dr. Robert Hessler referred to “a case of that extremely rare and almost extinct form of itch known as ‘Norway itch,’ the _scabies norvegica_ of Hebra, 1852.” The afflicted man was covered with thick, creamy white, leathery scales; some of these scales measured over an inch in diameter and 1/10 in. thick. A constant shedding of scales went on, a handful being gathered daily. They were found full of mites and eggs and riddled with passages. Under treatment the mites were killed and the skin became normal. Dr. Hessler made a calculation of the number of eggs and mites, amounting to ova and shells 7,004,000, mites in all stages 2,009,000.--F. V. T.]

The following forms may be transmitted from DOMESTIC ANIMALS to MAN:--

(1) _S. scabiei_ var. _equi_. Male, 0·2 to 0·23 mm. long, 0·16 to
0·17 mm. broad. Female, 0·40 to 0·42 mm. long, 0·28 to 0·32 mm.
broad. The horse is the normal host.

(2) _S. scabiei_ var. _ovis_. Male, 0·22 mm. long, 0·16 mm. broad.
Females, 0·32 to 0·44 mm. long, 0·24 to 0·36 mm. broad. This mite
lives on sheep, and passes over to goats and human beings; it may
also be artificially transferred to horses, oxen and dogs.[353]

[353] [This mite produces the so-called “black muzzle” of sheep.--F. V. T.]

(3) _S. scabiei_ var. _capræ_. Male, 0·24 mm. long, 0·188 mm. broad.
Female, 0·345 mm. long, 0·342 mm. broad. On goats, passing from them
to horse, ox, sheep, pig and man. On the latter, in contradistinction
to the varieties (1) and (2), it produces a severe affection.

(4) _S. scabiei_ var. _cameli_. Frequently observed in man, chiefly
in Africa. A few cases have been observed in Europe; the affection
induced by it is severe.

(5) _S. scabiei_ var. _aucheniæ_. Male, 0·245 mm. long, 0·182 mm.
broad. Female, 0·34 mm. long, 0·264 mm. broad. It lives on the llama,
and may be transmitted to man.

(6) _S. scabiei_ var. _suis_. Male, 0·25 to 0·35 mm. long, 0·19 to
0·3 mm. broad. Female, 0·4 to 0·5 mm. long, 0·3 to 0·39 mm. broad.
In the domestic pig and wild boar; occasionally also in man. The
settlement, however, is usually of short duration.

(7) _S. scabiei_ var. _canis_. Male, 0·19 to 0·23 mm. long, 0·14
to 0·17 mm. broad. Female, 0·29 to 0·38 mm. long, 0·23 to 0·28 mm.
broad. In the house-dog, and also, not unusually, in human beings.

(8) and (9) _S. scabiei_ var. _vulpis_ and _S. scabiei_ var. _leonis_
of the fox and lion have likewise been observed on man.

These are all distinct species and should read as follows: _S.
canis_, Gerl.; _S. ovis_, Mégn.; _S. equi_, Gerl.; _S. dromedarii_,
Gerv. (_cameli_, Mégn.); _S. aucheniæ_, Raill.; _S. suis_, Gerl.; _S.
vulpis_, Fürst.; _S. leonis_, Can.

*Sarcoptes minor*, Fürstenberg, 1861.

Anus situated on the back, legs short, pedunculated ambulacra broad; living on cats (_S. minor_ var. _cati_) and rabbits (_S. minor_ var. _cuniculi_). In cats this mite usually lives in the cervical region, and thence spreads to the ears and head; it usually causes the death of the infected animals; it is easily transferable from cat to cat, is difficult to transmit to rabbits, but once settled on them can easily infect other rabbits. On the other hand, the transmission of the itch mite of the rabbit to the cat does not succeed. In man _S. minor_ induces an eruption that disappears after about a fortnight.

[_S. minor_, Fürstenberg, 1861 (“Krätzm.,” viii, p. 218), comes in Railliet’s sub-genus NOTOEDRES, 1893 (“Zool.,” ed. 2, p. 660). Canestrini raised this to generic rank in 1894 (_Prosp. Acarof._, vi, p. 724).

[There are three species: (1) _N. notoedres_, Mégnin = _Sarcoptes alepis_, Railliet and Lucet (_Compt. rend. Soc. de Biol._, 1893, xlv, p. 404), and _Sarcoptes notoedres_ var. _muris_, Mégnin (_Parasitology_, 1880, pp. 172–174). This occurs on the black and brown rats and the water-vole.

[(2) _N. cati_, Hering, 1838 (_N. acta. ac. Leop._, ii, 18, xliv, p. 605, figs. 9 and 10), = _Sarcoptes minor_, Fürstenberg (“Krätzm.,” 1861, viii, p. 215). Found on the cat in Germany, France, Italy, and Britain.

[(3) _N. cuniculi_, Gerlach, 1857, “Krätzm.,” iii, figs. 20, 21. It lives on the rabbit and is found in Germany and France.--F. V. T.]

The itch mites of domestic animals, which belong to the genera
Psoroptes (= Dermatodectes = Dermatocoptes) and Chorioptes (Symbiotes
= Dermatophagus), as a rule do not infest and live on man, even when
artificially transmitted. It is, however, possible for this to occur.
Moniez (“Traité de par.,” 1896, p. 559) mentions that a species of
Chorioptes--probably _Ch. bovis_--had been found on man, as had also
_Demodex folliculorum_. This author also includes _Dermatophagoides
scheremetewskyi_, Bogdanoff (_Bull. soc. imp. d. natural._, _Moscou_,
1864, xxxvii, p. 341), which has repeatedly been found on man in
Moscow and Leipzig (Zürn, _Ber. d. med. Ges._, _Leipzig_, 1877,
p. 38), as _Chorioptes bovis_.

OTHER REFERENCES TO _Scabies crustosæ_ AND _norvegica_, ETC.

(1) “Ein Fall von _Scabies crustosa norvegica_,” _Würzb. med.
Zeitschr._, l, pp. 134–139, pl. 3, H. Bamberger.

(2) “Ueber die Krätzmilbe (_Acarus scabiei_),” _Notiz. a. d. Geb. d.
Nat. u. Heilk._, Weimar (1913), xlii (11), Oct., pp. 161–166 (1834),
de Blainville.

(3) “Rapport sur le ciron de la gale (_Acarus scabiei_),” _Ann.
de Mus. d’Hist. nat._, 1831; _Parasitology_, iv, pp. 213–232, de
Blainville.

Family. *Demodicidæ* (Mites of the Hair-follicles).

Small _Acarina_, elongated in worm-like fashion, with annulated
abdomen, and without eyes or tracheæ. The mouth parts consist of a
suctorial proboscis and three-jointed palpi; the legs are short, and
have three segments with small terminal ungues. The anus is situated
on the anterior border of the abdomen; oviparous; the larvæ have six
stumpy legs. These mites live in the hair-follicles of mammals.

Genus. *Demodex*, Owen.

*Demodex folliculorum*, Simon, 1842.

Syn.: _Acarus folliculorum_, Sim., 1843; _Demodex folliculorum_,
Owen, 1843; _Macrogaster platypus_, Miescher, 1843; _Simonea
folliculorum_, P. Gervais, 1844; _Steatozoon folliculorum_, Wilson,
1847.

As in _Sarcoptes scabiei_, numerous varieties of this species are known; the form parasitic on man lives in the hair-follicles, the meibomian and sebaceous glands, and hardly ever causes inconvenience; the male measures 0·3 mm. in length and the female about 0·4 mm. in length. The eggs 0·06 to 0·08 mm. in length, 0·04 to 0·05 mm. in breadth, and are thin-shelled. The creatures are always attached with the head end downwards in the parts mentioned; they are most frequent in the sebaceous glands of the face, by the nose, lips and forehead, but they may be present on the abdomen and on other parts of the body. They may occasionally obstruct the excretory gland ducts, thus causing inflammation of the gland (comedones); their agglomeration in the meibomian glands sets up inflammation of the margins of the eyelids. There are generally only a few specimens in a gland. According to some statements Demodex occurs in 50 per cent. of mankind and even in children; they survive the death of their hosts by several days.

The variety living in the dog (_D. folliculorum_ var. _canis_) is
smaller than the variety living in man, and produces a skin disease
resembling scabies in these animals. According to Zürn they may also
live on man; nevertheless, no other investigator has recorded a
similar observation, and attempts at artificial infection have proved
negative.[354]

[354] [This mite causes what we know in England as red mange in dogs.--F. V. T.]

[Ten distinct species of Demodex are given by Canestrini and Kramer (“Demodicidae und Sarcoptidae,” _Das Tierreich_, 1899, vii). The species are certainly distinct.

[The species living on the dog (_D. canis_, Leydig, 1844) is cosmopolitan. According to the _British Medical Journal_ (February 22, 1913, p. 407), dog mange may be caught by humans. Whitfield and Hobday describe in the _Veterinary Journal_ seventeen cases which have come under their observation.--F. V. T.]

Order. *Pentastomida.*

Family. *Linguatulidæ.*

_Arachnida_ greatly altered in consequence of their parasitic manner
of life; for a long time they were regarded as helminthes. The body
is elongated, vermiform, flattened or cylindrical, and more or less
distinctly annulated. The head, thorax, and abdomen are not defined
from each other (fig. 371). The elliptical mouth, surrounded by a
chitinous ring, is situated at the anterior end, on the ventral
surface, and the intestine leading straight through the body opens at
the posterior end. Two retractile hooks are at the sides of the mouth
(fig. 372); these are usually considered to be the terminal joints of
two pairs of legs, but it appears to be more correct to regard them
as the remains of the antennæ and palpi (Stiles). According to this
opinion, the legs in the adult state are completely degenerated.

The nervous system is reduced to an œsophageal ring. No organs of
sense are recognizable except the papillæ at the anterior end. There
are neither organs of circulation nor of respiration.[355]

[355] What are designated as stigmata in the Linguatulides are the orifices of sebaceous glands.

The sexes are distinct. In the small male the sexual orifice is
situated ventrally in the anterior part of the body; in the female it
is placed near the anus. The _Linguatulidæ_ lay eggs, and from each
egg, after being conveyed into an intermediate host, a four-legged
larva, with rudimentary mouth parts, hatches out. It goes through
a series of metamorphoses, and passes through a second larval
condition, which, however, possesses the essential characteristics
of the fully developed form. Sooner or later it migrates during this
stage, and reaches its final host, mammal or reptile, in the nostrils
or lungs of which the adult _Linguatulidæ_ live.

[As adults they live as internal blood feeders in various birds, reptiles and mammals, especially in the nasal and respiratory passages. The larval stage occurs in another host in an encysted condition; this host is usually an animal preyed upon by the species in which the sexual forms are found. The larvæ bore through the walls of the host’s stomach and enter liver and spleen or brain, where they encyst; here they grow until they assume almost the appearance of the adult. These encysted larvæ on being eaten later make their way into the nasal passages and lungs, where they mature. Both adults and larvæ occur in man, as mentioned later.

[Three genera are recognized in this family:--

[(1) _Linguatula._--Body flat, annulated. Adults live in the nasal
sinus.

[(2) _Porocephalus._--Body cylindrical, elongate, with often deeply
cut rings. Adult in respiratory organs of snakes, larvæ in animals
and man.

[(3) _Reighardia._--Cylindrical, but not ringed. Not found in
humans.--F. V. T.]

Genus. *Linguatula*, Fröhlich.

*Linguatula rhinaria*, Pilger, 1802.

Syn.: _Tænia rhinaria_, Pilger, 1802; _Polystoma tænioides_, Rud.,
1810; _Linguatula tænioides_, Lam., 1816; _Pentastoma tænioides_,
Rud., 1819.

The male is white in colour, 18 to 20 mm. in length, anterior portion 3 to 4 mm. in breadth, posterior part 0·5 mm. in breadth. The female is of a yellowish colour, 8, 10, or 13 cm. long, anterior part 8 to 10 mm. and posterior part 2 mm. wide. The brownish eggs can be seen in the median line. The body is elongated, rather flat, and exhibits about ninety rings or segments with crenellated borders. The hooks round the mouth are strongly curved and are articulated to a basilar support. Eggs oval, 0·09 µ in length, 0·07 µ in breadth.

_L. rhinaria_, in the adult condition, lives in the nasal cavity and frontal sinus of the dog, wolf, fox, horse, goat, and occasionally of man; it causes severe catarrh, epistaxis and suppuration.

_Development._--The ova, which are found in masses in the nasal mucus, already possess an embryo; they are expelled with the nasal secretion, and are swallowed by herbivorous mammals with their food, mostly by hares and rabbits, but also by sheep, goats, oxen, horses, antelopes, fallow deer, pigs, cats, and occasionally also by human beings. The young larvæ hatch out in the stomach; they possess a thickened anterior body with rudimentary mouth parts and two pairs of limbs; the body gradually tapers to a short tail.

The larvæ of the _Linguatulidæ_ bore through the intestinal wall and reach the liver, more rarely the mesenteric glands, etc.; they here become encysted and enter a sort of pupal stage in which they lose their limbs; after several moultings and gradual growth the second larval stage, having the appearance of the adult Linguatula, sets in. About five to six months after infection the creatures have become 4 to 6 mm. long, possess eighty to ninety rings, which have a series of fine points on their posterior border; the mouth and intestine are formed, the sexual organs mature and the two pairs of hooks are near the mouth. This larval stage (fig. 372) has been known for a long time, but it was regarded as an independent species of animal, and therefore had a separate name (_Linguatula serrata_, Fr.; _Pentastoma denticalatum_, Rud., etc.).

Later these Linguatula larvæ make an attempt to escape from their hosts, and this, of course, can only be effected by means of an active migration; they leave the cysts, and according to their respective positions in the abdominal or pleural cavities they reach the bronchi or the intestine, and finally pass out; they may be again sniffed up by dogs and settle in their nasal cavities. Still this outward migration does not appear to be necessary for further development. A portion of the larvæ gain access to the nasal cavities directly through the trachea, and thus herbivorous mammals certainly become directly infected. In most cases the infection of dogs, wolves and foxes, that is, of carnivorous mammals, takes place through consuming the bodies of mammals, or parts of them, such as the liver and lungs, which are affected with the second larval form; in any case most larvæ obtain access first to the stomach of their host, from here they make an active migration through the œsophagus to the oral and nasal cavities, in which they settle. It is possible also that the same larvæ which are free in the oral cavity when the food is being eaten migrate into the nasal cavities. After being stationary a fresh skin is formed and the spine-bearing cuticula are thrown off. The male attains its full size in the fourth, and the female in the sixth month. The duration of life is stated to be from fifteen months to several years.

_L. rhinaria_ has been observed in man in the adult as well as in the
larval condition (_Pentastoma denticulatum_). Zenker first called
attention to the occurrence of the larva in man, having found it
nine times in the liver in 168 autopsies. Heschl found it twice in
Vienna in twenty autopsies, Virchow found it in Würzburg and Berlin,
Wagner in Leipzig (10 per cent.), and Frerichs in Breslau five times
in forty-seven autopsies. The parasite is much less frequent in
Switzerland. According to Klebs, one case occurs in 900 autopsies,
and according to Zaeslin two cases occurred in Basle to 1,914
autopsies. In the Seamen’s Hospital in Kronstadt _P. denticulatum_
has been found six times in 659 autopsies. It was almost always the
liver that contained one or a few specimens. The parasite was very
rarely found in the kidney or spleen, or encysted in the intestinal
wall. The adult _L. rhinaria_ is far more rarely observed in man.

A case reported by Landon that related to a blacksmith of Elbing is
particularly interesting. This man accompanied the campaign of 1870;
he soon, however, fell ill with pains in the liver, accompanied by
icterus and intestinal disorders. Soon after the war, and after the
symptoms were reduced to icterus and weakness, bleeding of the nose
set in and continued with slight intermissions for seven years;
an unpleasant sensation of pressure in the left nasal cavity set
in, with inflammatory swelling of the mucous membrane. At last, in
the summer of 1878, when the pressure in the nose had considerably
increased, a Linguatula was expelled from the nose with a violent
attack of sneezing, and lived for three days longer in water. The
bleeding of the nose then ceased and the patient soon recovered.
There can be no doubt that the first illness was connected with
the invasion in the liver of numerous larvæ of Pentastoma, and
disappeared after their encystment; one or a few of these must
subsequently have found its way to the nose and settled there.

Genus. *Porocephalus.*

*Porocephalus constrictus*, v. Siebold, 1852.

Syn.: _Nematoideum hominis_, Diesing, 1851; _Pentastomum
constrictum_, v. Sieb., 1852; _Porocephalus constrictus_, Stiles,
1893.

Porocephalus is distinguished from Linguatula by its cylindrical body and by certain internal structures. _Porocephalus constrictus_ is at present only known in its larval stage. It is milk white in colour with golden-yellow hooklets. Number of rings, twenty-three. Length 13 mm., breadth 2·2 mm. There are no prickles on the posterior border of the annulations of the body.

This species was first discovered by Pruner encysted in the livers
of two negroes in Cairo. Bilharz reported two further cases in which
the parasites were encysted in the liver and in the mucosa of the
intestine; a few other observations have been made by Fenger, Aitken,
Giard and Chalmas. Aitken’s report deals with soldiers of the British
Colonies in Africa. The parasites were discovered in the liver as
well as in the lung, and appear to have been the cause of death in
one case (pneumonia, peritonitis).

Pruner has found the same parasite also in the liver of the giraffe.

It has recently been assumed that _Porocephalus constrictus_ is the
larva of _Pentastoma moniliforme_, Diesing, 1835, that attains a
length of 70 mm. and lives in the lungs of African Pythonides. The
larva is known to have been ejected from monkeys (_Cercopithecus
albogularis_, _Cynocephalus maimon_), from the giraffe
(_Camelopardalis giraffa_), from a species of hyæna (_Proteles
cristatus_), and should be expected to occur frequently in smaller
mammals which have been swallowed by African serpents of enormous
size.

[The three species of _Pentastomidæ_, or tongue worms, found in man are _Linguatula serrata_, Frölich; _Porocephalus armillatus_, Wyman; and _Pentastoma moniliformis_, Diesing.

[(1) _Linguatula serrata_ has been referred to under a great number of names.[356] It is a frequent parasite in dogs, oxen and sheep; as an adult in the dog and also in the fox and wolf. The nymphal stage is found in rats, hares, rabbits, the horse, oxen, sheep, goats, pigs, camels, deer, the African and long-eared hedgehogs, porcupine, guinea-pig and peccary. In man it is found in both adult and nymphal stages. Sambon says the nymphal stage is of frequent occurrence, but is usually overlooked. Zenker, who first found it in man, obtained it in nine out of 160 _post-mortems_, usually encysted in the liver. It is then said to be harmless. Landon, in 1878, found the adult in man, the patient suffering from epistaxis for about seven years; in the end during a fit of sneezing the living parasite was ejected through the nostril. This case is of particular interest as it appears to suggest that this Acarid may now and then pass its entire development in the same host, or at any rate may actively migrate from the liver to the nasal cavities after a period of encystment in the liver or elsewhere, which has also been observed in herbivorous animals (_vide_ also p. 526).

[356] Synonymy given by Sambon: Adult form, _Ténia lanceolé_, Chabert, 1787; _Ver rhinaire_, Chabert, 1787; _Tænia rhinaris_, Pilger, 1805; _Tænia lanceolata_, Rudolphi, 1805; _Cochlus rhinarius_, Rudolphi, 1805; _Prionoderma rhinaria_, Rudolphi, 1808; _Polystoma tænioides_, Rudolphi, 1809; _Linguatula tænioides_, Lamark, 1816; _Prionoderma lanceolata_, Cuvier, 1817; _Pentastoma tænioides_, Rudolphi, 1819; _Linguatula lanceolata_, de Blainville, 1828; _Linguatula rhinaris_, Railliet, 1885; _Linguatula caprina_, R. Blanchard, 1900. Nymphal form: _Linguatula serrata_, Frölich, 1789; _Tænia capræa_, Abildgaard, 1789; _Tænia caprina_, Gmelin, 1800; _Polystoma serrata_, Goeze, 1800; _Halysis caprina_, Zeder, 1803; _Linguatula denticulata_, Rudolphi, 1805; _Echinorhynchus capreæ_, Braun, 1809; _Tetragulus capriæ_, Bosc, 1810; _Pentastoma denticulatum_, Rudolphi, 1819; _Pentastoma emarginatum_, Rudolphi, 1819; _Pentastoma fera_, Creplin, 1829; _Linguatula ferox_, Gros, 1849.

[It is recorded from man in Central America (Darling, _Bull. Soc. Path. exot._, 1912, v, p. 118; and again _Arch. Int. Med._, 1912, v, p. 401), also from Rio de Janeiro (_Mem. Inst. Oswaldo Cruz_, 1913, fasc. ii, p. 125) by Faria and Travassos.

[(2) _Porocephalus armillatus_, Wyman, is also known under a variety of names.[357] This species is widely spread over tropical Africa. The adult stage is found in pythons and puff-adders, the nymphal in the chimpanzee, Sykes monkey, mandrill and other monkeys, the lion, leopard, banded ichneumon, Aard wolf, dog, black rat, South African reedbuck and the giraffe. The adult has never been found in man or any mammal. No fewer than sixteen cases of the nymphal form, Sambon tells us, have been found in man, and it is probably much more widespread than at present known. So far it has only been found in the African natives. This species has sixteen to seventeen body rings in the male, eighteen to twenty-two in the female, and the body does not taper as much as in the next species.

[357] Adult form as _Linguatula armillata_, Wyman, 1847; _Pentastomum polyzonum_, Hailey, 1856; _Pentastomum armillatum_, Leuckart, 1860; _Pentastomum armillatum_, Diesing, 1864; _Porocephalus armillatus_, Stiles, 1893; _Porocephalus polyzonus_, Stiles, 1893; _Porocephalus moniliformis_, Neumann (in part), 1899. Nymphal form: _Linguatula diesingii_, van Beneden, 1849; _Pentastomum euryzonum_, Diesing, 1850; _Nematoideum hominis_, Diesing, 1851; _Pentastomum constrictum_, von Siebold, 1852; _Linguatula constricta_, Küchenmeister, 1855; _Pentastoma leonis_, Wedl., 1863; _Pentastoma fornatum_, Cobbold, 1879; _Pentastomum protelis_, Hoyle, 1883; _Porocephalus constrictus_, Stiles, 1893; _Linguatula constrictor_, Galli-Valerio, 1896; _Pentastomum diesingii_, Shipley, 1898.

[(3) _Pentastoma moniliformis_, Diesing,[358] is an Oriental species, found in India, Indo-China and South China, and the Malay Archipelago. The adult occurs in both the Indian and reticulated pythons. The nymphal stage has been found in monkeys, the tiger, the civet and the Indian otter.

[358] The synonymy is as follows:--Adult form: _Pentastoma moniliforme_, Diesing, 1835; _Linguatule moniliforme_, Mégnin, 1880; _Porocephalus moniliformis_, Stiles. Nymphal form: _Pentastoma fornatum_, Creplin (in part), 1849; _Pentastoma wedlii_, Cobbold, 1866; _Pentastoma aonycis_, Macalister, 1874; _Porocephalus armillatus_, Stiles (in part), 1908.

[The nymph has twice been found in man; in one case in the liver of a Filipino, the other in the serous coat of the small intestine of a native of Sumatra.

[This species can be told by the female having twenty-nine to thirty-three body rings, the male twenty-six, and the annulations are more bead-like and less prominent than in the African species.

[In addition to these three, Sambon thinks it probable that others occur in man.--F. V. T.]

OTHER REFERENCES TO _Pentastomidæ_.

(1) “Die Wanderung des _Pentastomum denticulatum_ beim Rinde,”
_Centralbl. f. Bakt. u. Parasitenk._, Jan. 2, 1889, v (1), pp. 1–5,
V. Bates.

(2) “Il _Pentastoma moniliforme_, Dies., nella pantera,” _Med.-vet.
Torino_, 1877, 4 s., vi (12), pp. 529–532, R. Bassi.

(3) “On the Organization and Development of Linguatula (Pentastoma),
accompanied with the description of a new species from the abdominal
cavity of the mandrill,” _Ann. and Mag. Nat. Hist._, 1848, 2 s. ii
(7), 2, pp. 69–70, v. Beneden.

(4) “De la _Linguatula ferox_ (_Pentastoma denticulatum_ aut
_serratum_),” _Bull. Acad. roy. d. Sci. d. Belg._, 1855, xxii, pt. 1
(1), pp. 4–10, v. Beneden.

(5) “Note sur quelques pentastomes,” _Bull. Acad. roy. d. Sci. de
Belg._, 1857, 26, 2 s., ii (5), pp. 29–30, v. Beneden.

(6) “Ueber das _Pentastoma_ in de gekrösdrusen den Schafe,” _Repert.
d. Thierh. Stuttg._, 1861, xxii, pp. 37–38, Collin.

(7) “Eine Linguatula aus der Mesenterialdrüse des Schafes und
Dromedars als zweites ungesche. Stadium von _Pent. taenioides_,”
_Notiz. u. Tagsber. u. d. Geb. d. Nat. u. Heilk._ Jena, 1862, v,
pp. 127, 128, Colin.

_B._ *INSECTA* (_Hexapoda_).

Three separate regions can always be distinguished in the body
of insects, namely, the head, thorax and abdomen. The HEAD is a
roundish unsegmented capsule and possesses four pairs of appendages.
The first pair are the various shaped feelers (antennæ), which are
placed on the superior surface of the head next to the eyes; then
more ventrally placed a pair of upper jaws (mandibles) without
palpi and without articulations; they are powerful masticatory
organs.[359] The first pair of lower jaws (maxillæ) are jointed and
bear a palpus (palpus maxillaris); the second pair of maxillæ are
soldered together and form the lower lip (labium), and likewise carry
a palpus labialis on each side. The upper lip (labrum), as well as
the other parts (which, however, are only appendages), belong to the
mouth, which is really formed of a number of closely united pieces.
The mouth parts are modified according to the functions required
of them. _Coleoptera_, _Neuroptera_, and _Orthoptera_ have biting
or masticatory mouth parts which conform with the scheme described
above. In the licking mouth parts of the _Hymenoptera_ the maxillæ
and under lip are considerably elongated, while the mandibles
retain their form and are used for triturating the food; in the
_Lepidoptera_ nearly all the mouth parts are shortened except the
maxillæ, which form a long and sometimes spirally rolled suctorial
proboscis; the _Diptera_ and _Rhynchota_ have piercing and sucking
mouth parts. The mandibles and maxillæ are metamorphosed into
needle-like structures, while the suctorial apparatus is formed by
the labrum.

[359] [The mandibles are only powerful masticatory organs in biting-mouthed insects (_Mandibulata_); in the sucking or piercing-mouthed insects they may be absent, or in the form of needle-like stylets (_Haustellata_).--F. V. T.]

The thorax consists of three segments, which are frequently united;
ventrally it carries three pairs of legs, which consist of a definite
number of articulated pieces joined together. Their form also changes
according to their function, so that legs for running, walking,
digging, swimming, jumping, and preying are seen. A pair of wings are
respectively attached to the last and last but one thoracic rings,
and these may be traced back, not to metamorphosed appendages, but to
tracheal branchia. They are composed of chitinous membranes supported
by branched structures (veins or ribs). Their size and formation
vary; they are seldom of equal size and form (_Neuroptera_);
often the posterior wings are larger than the anterior wings, the
former then only serving as protective coverings for the latter
(_Coleoptera_), or the anterior wings are larger (_Lepidoptera_), or
the posterior wings are shortened or are entirely absent (_Diptera_);
and finally there are insects in which both pairs of wings are
lacking.[360]

[360] [As in the order _Aptera_, which includes the Thysanura and Collembola, and also exceptions in other orders, as the fleas amongst _Diptera_, the Mutillus and ants amongst _Hymenoptera_.--F. V. T.]

The abdomen retains its segmentation, but, with the exception of
a few groups related to the primitive forms of insects, has no
appendages in the imago condition; the abdomen usually consists of
ten segments, on the last of which the anus is situated.

We need only observe the following characters in considering the
anatomy of insects:--

The EPIDERMIS consists of the chitinous cuticle, which is separate
from the cellular layer beneath (hypodermis); the various appendages
are supported by the chitinous layer.

The INTESTINAL CANAL usually consists of the anterior, median and
terminal intestine, and as a rule passes straight through the body;
salivary glands discharge into the anterior part, and, in some cases,
yield a stiffening secretion which serves for spinning webs; numerous
or scanty hepatic tubes are appended to the median intestine, while
on the border between the median and terminal intestine open four
to six long tubes (vasa malpighiana), which act as urinary organs.
Finally the end portion of the intestine carries various glands (anal
and rectal glands, etc.).

The CENTRAL NERVOUS SYSTEM agrees in structure with that of the
Annelids, but is more highly developed. The pharyngeal ring surrounds
the front part of the intestine; the sensory nerves originate from
its SUPERIOR PHARYNGEAL GANGLIA and are the seat of the higher
psychical functions; the INFERIOR PHARYNGEAL GANGLIA govern the mouth
parts, and in addition appear to regulate the movements (cerebellum)
of the vertebrates.

The chain of GANGLIA lying on the ventral side of the abdomen
consists primitively of pairs of ganglia corresponding with the
twelve segments, which are connected by longitudinal and transverse
commissures. But many changes in the ganglia may be seen in insects
caused by partial or entire amalgamation of single ganglia, so that
in a few cases only one abdominal ganglion is present. In conclusion,
a definite INTESTINAL NERVOUS SYSTEM is always present.

Of the organs of sense the FACETTED EYES, situated at the sides of
the head, deserve special mention, as do also the ORGANS OF TOUCH and
SMELL, situated on the antennæ, and the ORGANS OF HEARING and taste,
or finer sensations, situated at the mouth and in the buccal cavity.

The sounds emitted by insects are, as a rule, produced by the
friction or beating of certain chitinous parts, but sounds are also
produced in breathing (flies).

The ORGANS OF RESPIRATION, the so-called tracheæ, are highly
developed; there are openings (stigmata) at the sides of the body
which draw in air by means of the active participation of the muscles
of the body. The number of stigmata varies between two and ten pairs;
the tracheæ themselves branch off from the trunks in the most varied
manner, and carry air to the internal organs.

The colourless BLOOD circulates between the tissues and organs, and
is kept circulating by the contraction of a chambered dorsal vessel
provided with ostia, and which terminates in a short aorta opening at
the anterior end.

Insects are SEXUALLY DISTINCT; their sexual glands are in pairs and
have a tubular structure, but the testicular tubules are united
together by a capsule into an oval testicle; exceptionally, also, the
excretory canals are double, as also the sexual orifices; usually the
paired canals unite into a single oviduct or spermatic duct which
terminates at the posterior end of the body after receiving the
products of various glands.

As to the HISTORY OF THE DEVELOPMENT of insects, all that is
necessary to mention here is that the young hatched from eggs only
exceptionally (as in _Apterygota_) resemble the adult parent (insecta
ametabola); as a rule they differ from them not only in the shape
of the body, but also more or less by their manner of life, and
only attain the form of the parent through METAMORPHOSIS. This is
a gradual process (insecta hemimetabola) in the _Rhynchota_ and
_Orthoptera_, or a sudden one with a stage of inanition (insecta
metabola) in the other orders. This stage of rest or inanition, the
PUPA, concludes the larval life (caterpillar, maggot, etc.); during
the pupal stage no nourishment at all is taken, but the internal
organs undergo changes; in some forms the rest is not absolute, as
voluntary local movements may take place (pupæ of gnats).

The insects are divided into numerous orders according to the form
of the mouth parts, the structure of the wings, as well as the
manner of the development; with the exception of the lowest group
(_Apterygota_), which is most nearly related to the ancestors of the
insects, and which has no wings and undergoes no metamorphosis, all
the remaining orders, which are termed _Pterygota_, have wings on the
thorax, though there are, of course, a few species and families of
this group which have lost their wings.

The _Pterygota_ include--

(1) _Orthoptera._--Biting mouth parts, anterior wings leathery,
posterior wings thin, folded longitudinally; metamorphosis incomplete
(grasshoppers, crickets, cockroaches).

(2) _Pseudoneuroptera._--Biting mouth parts, wings of equal size,
thin, not folded up (dragon-flies, hair and feather lice, termites).

(3) _Rhynchota_ or _Hemiptera_.--Mouth parts formed for puncturing
and sucking; wings alike, or the anterior wings may be thickened,
parchment-like at their base (plant lice, cicadæ, bugs and true lice).

(4) _Neuroptera._--Biting mouth parts; wings alike, thin;
metamorphosis complete (ant-lions, lace-wing flies, etc.).

(5) _Trichoptera._--Licking mouth parts; anterior wings narrow,
posterior wings longitudinally folded, both ornamented with little
hairs; the larvæ are worm-like in form, live in water, and breathe
through tracheal gills (may flies, etc.).

(6) _Lepidoptera._--Suctorial mouth parts; wings covered with scales
(butterflies).

(7) _Coleoptera._--Biting mouth parts; anterior wings thickened and
differ in colour, appearance and function from the thin, folded
posterior wings (beetles).

(8) _Hymenoptera._--Mouth parts for licking and biting; the wings
alike, membranous (ichneumon flies, ants, wasps, bees, humble bees).

(9) _Diptera._--Mouth parts formed for puncturing, sucking or
licking; posterior wings degenerated (gnats, flies, gadflies, fleas).

(10) _Strepsiptera._--Anterior wings shortened; the female without
wings and living parasitically (fan-wings).

The parasites of man occur amongst the _Rhynchota_, _Coleoptera_, and amongst the _Diptera_.

[The most usual and recent classification of the _Hexapoda_ is the
following:--

(1) _Aptera._--Wingless insects; scarcely any metamorphosis.

(2) _Neuroptera._--Four membranous wings, frequently with much
network; the front pair not much, if at all, harder than the under
pair; the latter with but little or no fan-like action in closing;
mandibulate; metamorphosis variable, but rarely complete.

(3) _Orthoptera._--Four wings; front pair coriaceous or leather-like,
usually smaller than the other pair, which are of more delicate
texture and contract in repose like a fan; mandibulate; metamorphosis
complete.

(4) _Thysanoptera._--Four very narrow fringed wings; mouth
imperfectly suctorial; metamorphosis slight.

(5) _Hemiptera._--Four wings; the front pair either all transparent
or with the basal half leathery; mouth suctorial; metamorphosis
slight.

(6) _Diptera._--Two membranous wings only; mouth suctorial, very
varied; metamorphosis complete.

(7) _Lepidoptera._--Four large wings covered with scales; mouth
suctorial, metamorphosis great.

(8) _Hymenoptera._--Four membranous wings; front pair larger than
hind, which do not fold up in repose; mandibulate, sometimes with a
tubular proboscis; metamorphosis complete.

(9) _Coleoptera._--Four wings, the front pair hard and horny
(elytra), meeting in a line over the back and covering the delicate
hind pair; mandibulate; metamorphosis complete.

[There are two other well-known arrangements, namely, Packard’s and
Brauer’s, of recent date, but the one given here, which is based on
Linnaeus’ grouping by Dr. Sharp, is by far the simplest.--F. V. T.]

Order. *Rhyncota.*[361]

[361] [Usually known as _Hemiptera_. There are two sub-orders, _Heteroptera_ and _Homoptera_. The former have the base of the front wings coriaceous; the latter have all four wings membranous. The _Homoptera_ are Aphides or plant lice and scale insects (_Coccidæ_), none of which attack man. Recently an interesting case has been reported to me where certain Aphides had been passed in human urine. One species was _Rhopalosiphum dianthi_, the other found in the urine was the hop aphis (_Phorodon humuli_). I cannot believe, however, that they had been actually passed, in spite of the case being reported by a medical man.--F. V. T.]

The lower lip forms a long thin tube that can be turned back
(rostrum), and within which lie the setaceous mandibles and maxillæ;
the first thoracic segment is not united with the two posterior ones;
the anterior wings are usually leathery as far as the centre.

(_a_) RHYNCOTA APTERA PARASITICA.

Family. *Pediculidæ* (Lice).

The lower lip is transformed into a projecting rostrum provided with
barbed hooklets in which the hollow extensile sucker (maxillæ and
mandibles) is situated; no wings; no metamorphosis; only simple eyes;
the antennæ are five-jointed, the feet possess hook-like terminal
structures; the barrel-shaped eggs (nits) are deposited on the hair
of the host.

[The lice or _Pediculidæ_ are also known as _Anoplura_ and
_Siphunculata_.

[They have been split up into a number of families and sub-families
and a number of genera, but as far as this work is concerned it is
best to retain the single family _Pediculidæ_.

[Only the three species mentioned here are common parasites of man,
but now and then horse and cattle and sheep lice, _Hæmatopinus_, may
cause transitory annoyance.--F. V. T.]

Genus. *Pediculus*, Linnæus.

*Pediculus capitis*, de Geer, 1778.

Male 1 to 1·5 mm. in length, female 1·8 to 2·0 mm. in length. The colour varies from light grey to black according to the colour of the hair of the human race upon which they are parasitic. The abdomen has eight segments, of which the six central ones are each provided with a pair of stigmata. The thorax is as broad as the abdomen. Eggs 0·6 mm. in length; about fifty are deposited by a female head louse. The young can propagate when eighteen days old.

The head louse lives especially in the hairy parts of the head
of human beings; more rarely it is found on other hairy parts of
the body. It is spread over the entire surface of the globe, and
was present in America before the arrival of Europeans. Quite
exceptionally it is said that it bores itself deep into the epidermis
and can live in ulcers.

[The eggs are pear-shaped and are attached to the hairs near the
roots by means of a clasping collar. They hatch in about seven
days. The young are like the adults and mature in a month. Its
general colour varies with that of its host. In West Africans
nearly black, in Hindoos dark and smoky, on Chinese and Japanese
yellow, on Hottentots orange, on South American Indians dark brown
(Murray).--F. V. T.]

*Pediculus vestimenti*, Nitzsch, 1818.

The head in front is somewhat rounded. Antennæ longer than in the head louse; 2 to 3·5 to 4 mm. in length; whitish-grey; the abdomen is broader than the thorax; stigmata as in _P. capitis_. Eggs 0·7 to 0·9 mm. in length; about seventy are deposited.

_P. vestimenti_ lives on the neck, throat and trunk of persons, and
the clothing next the body, in which also the eggs are deposited.
The louse of so-called pedicular disease (_P. tabescentium_) is,
according to Landois’ researches, only the usual _P. vestimenti_;
moreover, many cases of phthiriasis are attributable to mites or fly
maggots.