Chapter IV: Part 4

The mandibles scarcely reach halfway down on the anterior coxæ; upon each are two large spines; the pronotal comb has about nine spines each side; and each abdominal segment has but a single row of bristles. The hind femora have six to eight bristles on the side; the proportions of the joints in the hind tarsus are: 26–16–8–5–13. Color, dark reddish brown. Female, 2.5 millimeters; male, 1.5 millimeters.

Described from a spermophile from Colorado and recorded by Doctor Fox and Professor Doane from _Mus norvegicus_ from California.

_Pulex._—Of this, the typical genus of the family, but one species has been recorded from rats.

_Pulex irritans_ Linn.

The mandibles reach about halfway down on the anterior coxæ; the head is regularly rounded in front; there are no transverse rows of bristles on the vertex, and but one row of bristles on each abdominal tergite. The proportions of the joints in the hind tarsus are, 50–30–18–12–32. Color, usually yellow brown. Male, 1.6 to 2 millimeters; female, 2 to 3.5 millimeters.

This, the human flea, is quite cosmopolitan, but more abundant in warm countries than elsewhere. It occurs on many domestic animals and has frequently been taken from rats in California and elsewhere; it also occurs on skunks.

_Xenopsylla._—This genus includes the following species, formerly placed in the genus _Lœmopsylla_.

_Xenopsylla cheopis_ Rothschild.

The mandibles reach nearly to the end of the anterior coxæ; there are no ctenidia on the head or pronotum; the eyes are distinct; each abdominal tergite has but one row of bristles; the hind femur has a row of about eight bristles; the proportions of the joints in the hind tarsus are as follows: 46–30–16–10–20. Color, light brown. Male, 2.5 to 3.5 millimeters; female, 4 to 5.5 millimeters.

This is a true rat flea, but will readily bite man, and is the species chiefly concerned in transmitting the bubonic plague. It is widely distributed, especially in seaport towns.

_Ceratophyllus._—Fleas of this genus are abundant on many kinds of small mammals, especially rodents. There are a great many species and some are so closely related that it is not easy to identify them. Of the eight species recorded from rats, four have been taken in this country. It is not practicable to tabulate these eight species, but the four that occur in our country may be arranged as follows:

1. Hind tarsal joint II with an apical spine much
longer than joint III _acutus_.

Hind tarsal joint II with spines not longer than
joint III 2

2. Pronotal comb of about 26 spines _niger_.

Pronotal comb of about 18 or 20 spines _fasciatus_ and
_londiniensis_.

_Ceratophyllus niger_ Fox.

This species has the pronotal ctenidia of about 26 spines; there are a few hairs on the inner surface of hind femur; apical spines of second joint of hind tarsus not longer than third joint; three hairs in front of the eye and three in front of these; movable finger of claspers with five slender bristles on the outer edge. Color, very dark brown. Length 3.5 millimeters.

Taken in California from _Mus decumans_ and from man.

_Ceratophyllus acutus_ Baker.

This species is readily known by having a spine at tip of the second joint of hind tarsus longer than the third joint and reaching over onto the fourth joint; the abdominal tergites have each two rows of bristles; the male claspers are very large and long, sickle shaped. Color, pale brown. Length, 3 to 3.5 millimeters.

It was described from a spermophile, but Doctor Fox has taken it once from a rat in California.

_Ceratophyllus fasciatus_ Bosc.

There are 18 or 20 spines in the pronotal comb; there are three bristles in front of eye and in female two, and in male four in front of these; there are three or four hairs on the inner surface of the hind femur; the proportions of joints in the hind tarsus are 50–33–20–11–21. The manubrium of the male claspers is very long and slender, and some of the bristles on the movable finger are as long as the joint. Length, male, 1.8 millimeters; female, 2.5 millimeters.

It has been recorded from California on rats, mice, skunks, and man. It is also common in Europe and elsewhere on rats, mice, and other small animals.

_Ceratophyllus londiniensis_ Rothschild.

This is allied closely to _C. fasciatus_, and is best separated from that species by the shape and armature of the genital parts; the manubrium is not as long as in that species, and the bristles on the movable finger are shorter; the third joint of the maxillary palpi is proportionally longer than in _C. fasciatus_. There are three bristles in front of the eyes and four or five in front of these; there are a few hairs on the inner surface of the hind femur; the proportions of the joints in the hind tarsus are 46–30–18–11–18.

It has been recorded by Doctor Fox from _Mus rattus_ in California, and is known from rats and mice from several parts of Europe; the _C. italicus_ Tiraboschi is the same species.

The four other species of this genus found on rats and not yet found in our country are closely related to _C. fasciatus_, and distinguished chiefly by the shape of the male genitalia.

_Ceratophyllus mustelæ_ Wagner.

This species has no series of hairs on the inner surface of the hind femur; there are three bristles in front of the eye and six in front of these; the pronotal comb has 18 or 20 spines; the proportions of the joints in the hind tarsus are 47–37–20–13–20; the movable finger of the male clasper has a long process below not seen in other forms. Occurs (according to Rothschild) on rats in Europe.

_Ceratophyllus pencilliger_ Grube.

This species also has no hairs on the inner surface of the hind femora. The pronotal comb has 18 spines; there are three bristles in front of the eye and four in front of these; the proportions of the joints in the hind tarsus are 52–36–23–14–24; the outer corner of the movable finger of the male clasper has two little rounded processes. It was described from Siberia, but according to Rothschild occurs on rats in Europe.

_Ceratophyllus consimilis_ Wagner.

This species is very close to _C. fasciatus_, and has some fine hairs on the inner surface of the hind femur; there are but two bristles in front of the eye and in front of these a few finer hairs; the proportions of the joints in the hind tarsus are 42–30–20–11–19; pronotal spines 18. Occurs on rats in Russia.

_Ceratophyllus lagomys_ Wagner.

This species also has a few fine hairs on the inner surface of the hind femur; 18 spines in pronotal comb; there are three bristles in front of eye and one in front of these; the proportions of the joints in the hind tarsus are 53–32–20–11–22; the outer corner of the movable finger of the male clasper has two little processes, similar to those on _C. pencilliger_. Occurs on rats in Europe.

_Ctenocephalus._—The common fleas on cats and dogs, as well as on man, belong to two species long kept under one name (_C. canis_ or _C. serraticeps_), but lately shown by Rothschild to be distinct. Both have a comb of 8 spines on the head and 16 spines in pronotal comb; the proportions of joints in the hind tarsus are 40–24–15–10–24. Both are occasionally taken on rats in this country. They may be separated as follows:

1. In the female the head is fully twice as long as
high (seen from side); the first spine of genal
comb is two-thirds the length of the second; in
male the manubrium of claspers is barely enlarged
at tip; and with two rows of hairs on disc of _C. felis_
movable finger Bouché.

In the female the head is less than twice as long
as high (seen from side); the first genal spine
in the head comb is only about one-half the
length of the second; in the male the manubrium
of clasper is very distinctly enlarged at tip;
but one row of hairs on the disc of the movable _C. canis_
finger Curtis.

_Neopsylla._—One species of this genus has been described from the brown rat in Europe.

_Neopsylla bidentatiformis_ Wagner.

The eyes are very small; there are 4 pairs of lateral spines beneath the last joint of the hind tarsus; the comb on head consists of but 2 stout spines, below the middle of the antennæ; the pronotal comb has 18 spines; the proportions of the joints in the hind tarsus are 43–33–21–12–21.

Length: Male, 2 to 2.3 millimeters; female, 2.3 to 2.5 millimeters.

Not yet found in the United States; described from Russia.

SARCOPSYLLIDÆ.

The fleas of this family are commonly called “chigoes,” “jiggers,” or sand fleas. The head is usually larger proportionally than in the other fleas; there are no ctenidia on head or pronotum; the thoracic segments are extremely short, and in the female the abdomen enlarges with the development of the eggs. They do not hop about as other fleas, but remain on the spot to which they have attached until they die. Frequently the adjacent skin grows over them, forming a swelling of considerable size.

Three species belonging to two genera have been recorded from rats.

1. Angle of head acutely produced; fifth tarsal joint
of hind legs without heavy spines; few spines on
the legs _Sarcopsylla_.

Angle of head not produced, obtuse; fifth tarsal
joint with heavy lateral spines, and other spines
on other parts of the legs _Echidnophaga_.

_Echidnophaga._—Two species of this genus are known from rats; one, however (_E. gallinacea_), can hardly be called a normal parasite, but rather of accidental occurrence. The genus has also been called _Argiopsylla_ and _Xestopsylla_.

1. Bristles at end of second joint of hind tarsus
about as long as next three joints; palpi about _E.
one-half the length of mandibles rhynchopsylla_.

Bristles at end of second joint of hind tarsus
about as long as next two joints; palpi about
two-thirds the length of the mandibles _E. gallinacea_.

_Echidnophaga rhynchopsylla_ Tiraboschi.

The body is about twice as long as broad and shining brown; there is but one hair in front of eye, and four on each metathoracic pleuron; mandibles larger than in _E. gallinacea_, and the spiracles are much higher up on the sides than in that species. Length, 1.4 to 1.8 millimeters.

Taken from _Mus rattus_ in Italy.

_Echidnophaga gallinacea_ Westwood.

This species has the body almost as broad as long, and of a red-brown color; 1 bristle in front of eye and 6 on each metathoracic pleuron; each abdominal tergite has on each side near the median line a single hair; the spiracles are situated well down on the sides. Length: Male, 0.8 to 1.2 millimeters; female, 1 to 1.8 millimeters.

This species is a fairly common pest of poultry and dogs in warm countries, and is called the “chicken flea.” It has been taken from rats in Italy.

_Sarcopsylla._—This genus includes the _S. penetrans_, which attacks the feet of various animals, including man, in the Tropics. This species has not yet been recorded from rats, but an allied species is described from Brazilian rats.

_Sarcopsylla cæcata_ Enderlein.

Color, clear yellowish. Eyes rudimentary; lower anterior corner of coxæ prolonged in a tooth; tarsal joints very short; claws long, but little curved, and almost hair like. The body of a swollen female is about 5 millimeters long.

Taken from _Mus rattus_ in Brazil.

LICE—ANOPLURA.

The insects known as Pediculi, or lice, are parasitic during their entire life on various mammals, including man. They are flat, rather elongate, wingless insects, with a small head and stout legs, which end in a strong claw, opposable to a projection at the tip of the penultimate joint. The simple antennæ, three to five jointed, are inserted in a concavity on the side of the head. The mouth parts are of a very peculiar nature, and not yet homologized with the cibaria of other insects. There is a short beak or proboscis in front, with recurved spines or hooks on its dorsal and lateral surfaces. Through this beak extends a slender stylet, that is formed of three parts; a ventral channeled piece, perhaps a labium; a dorsal piece, consisting of two pieces fused together, perhaps the maxillæ; and a median tube, possibly the hypopharynx. The stylet is used to pierce the skin of the host, and the blood is sucked up through it. There are no palpi. On each side of the head there is a small, simple eye. The thorax shows only incompletely the division into the three parts; there is a large spiracle above on each side. The abdomen shows eight segments, six of them have a spiracle, or breathing pore, on each side, the basal and apical segments being without them. All of the segments bear a few simple hairs or bristles; the longest are on the posterior segments. The legs are stout and prominent; they consist of a broad coxa, a small trochanter, a longer femur, a tibia with an apical process, and a tarsus of one joint and a very large terminal claw. At the apex of the tibia, just within the projection, is a sucking disc. This, the projection, and the claw form the apparatus to hold fast to the hair of the host.

Lice usually walk sideways, but do not travel much, and they keep close to one host. The eggs are slightly elongate and fastened to the hair of the host. They hatch in about ten to fifteen days, the young coming out of the top of the egg. These young do not differ much in structure from the adults, but are paler in color. They molt their skin a few times, probably four, before they reach the mature condition. The males are less numerous than the females, and ordinarily smaller. There are several generations each year, dependent doubtless on the temperature; but the life history is not thoroughly known for any species. After sucking the blood the abdomen of the louse becomes somewhat distended, very noticeably so in some species.

The sucking habits of the lice render them dangerous parasites and capable of transmitting a disease from one host to another. Fortunately they do not readily change hosts so that they can not be considered quite as dangerous as some more active parasites. However, several species have already been shown to carry diseases in laboratory experiments. Therefore it is probable that some of them will be connected with the origin and diffusion of certain diseases of animals.

FIG. 7.—Louse (_Polyplax spinulosa_).
]

The Anoplura, or lice, have often been treated in connection with the Mallophaga, or biting lice. This is doubtless because they frequently occur on the same animal, and have a general resemblance to them. However, they have no real affinity to these insects, and the general opinion is that they are more or less related to the Hemiptera. Sometimes they are treated as a group or section of the Hemiptera, but also as a separate order, under various names as Siphunculata, Lipognatha, Pseudorhynchota, and Ellipoptera.

There are about 50 or 60 known species which are arranged in 15 genera and 4 families. Four species belonging to three different genera have been recorded from rats; a number of others are known from mice and other rodents, and some of these will probably yet be taken upon rats.

These four forms are separated, as follows:

1. Eyes large and distinct; beak very short; thorax _Pediculus
plainly longer than broad capitis_.

Eyes small, beak longer; thorax about as broad as
long 2

2. In male the pleura of abdominal segments 3 to 6
above and below have a prominent tooth-like _Hoplopleura
projection; a tooth on the hind femur of female acanthopus_.

In male the pleura of abdominal segments without
such projections; no tooth on hind femur of
female 3

3. Last joint of antennæ much more slender than those
before; an acute tooth at sides of segments 4 to
7; head much narrower in front; antennæ _Polyplax
two-thirds as long as head miacantha_.

Last joint of antennæ not much more slender than
others; head quite broad in front; antennæ as _Polyplax
long as head spinulosus_.

_Pediculus capitis_ De Geer.

It is pale grayish in color, with faint dark markings at the sides of the thorax and abdomen; the last segment of the abdomen in female is bilobed. The head is longer than broad and tapers in front. Length, 2 millimeters. This is the head louse of man, and is said to have been taken from rats, and is claimed to be able to transfer plague from rats to man. Its occurrence on rats, however, appears to be very uncommon.

_Hopopleura acanthopus_ Burmeister.

In the male the pleura of the abdominal segments 3 to 6, which reach up on the dorsum and over on the venter, have at their inner ends a prominent projection, toothed in all except the third on dorsum and the sixth on venter, which are spine-like. The head is but little longer than broad, broad in front; and in the female there is a recurved tooth on each hind femur. The last segment of the female abdomen is bilobed behind. Length, 1.3 millimeters. It has been taken from rats in Europe, but is more common on species of _Microtus_.

_Polyplax spinulosus_ Burmeister.

The sides of the abdominal segments are acute, but the males do not have the large tooth-like projection of _Hoplopleura_. The last segment of the female is truncate; the head is about as broad in front as behind, and the legs are very short and stout; the antennæ are as long as head, and the last joint is but little smaller than the others. Color, pale yellowish. Length, 1.4 millimeters.

This is the common rat louse, and is probably as widely distributed as its host. Specimens have been taken in both the eastern and western parts of the United States.

_Polyplax miacantha_ Speiser.

This differs from _P. spinulosus_ in having a longer and narrower anterior part of head, in that the last joint of the antennæ is more slender, and the antennæ are only two-thirds as long as the head. The abdominal segments 4 to 7 show an acute process at the sides. Length, 1.5 to 1.75 millimeters.

Taken from rats in Abyssinia.

MITES—ACARINA.

The mites (order Acarina, class Arachnida) are readily known from the insects (fleas and lice) by having four pairs of legs, no antennæ, and the abdomen does not show any segmentation, nor is there usually any distinction between head and thorax. In some groups there is a small head-like part, called the capitulum. The mouth parts consist of a pair of mandibles (often styliform or needle-like), a lip, and a pair of palpi. In some forms there is a central piece, called the hypopharynx, and in other groups is a plate above the mouth parts, known as the epistome. The body usually shows more or less distinctly a division into two parts—the anterior, called the cephalothorax, and the posterior the abdomen. However, in many mites it is not possible to separate these parts, except that it is considered that the legs are borne by the cephalothorax. In many forms there is a small, simple eye each side on the cephalothorax, but many other forms are blind. Some species have a tracheal system, which opens in a pair of spiracles near the hind legs or near the anterior end of the body; other species have no definite respiratory system. The genital aperture is on the venter, usually between the legs. The legs consist of the usual joints—coxa, trochanter, femur, tibia, sometimes a metatarsus, and a tarsus. The tarsus terminates in a pair of claws, sometimes three or only one, and often a sucker or caroncle. Most mites are not parasitic; those species that are parasitic are often free in one stage. The parasitic mites suck the blood of their host, feed on the hair or dermal scales, or burrow in the skin. Some predaceous species inhabit animals to hunt and eat the parasitic mites that infest that animal.

The mites that occur parasitically on rats belong to four families: Sarcoptidæ, Cheyletidæ, Ixodidæ, and Gamasidæ.

1. A distinct spiracle or breathing pore on each side
of body near coxæ III or IV 2

No such spiracle or pore visible 3

2. A small, distinct head part in front of the body;
palpi three jointed; a granulate area around the
spiracle; no sternal plate _Ixodidæ_.

No such head part; palpi five jointed; no granulate
space around spiracle, but a long, chitinized
piece reaching forward from it; a more or less
distinct sternal plate _Gamasidæ_.

3. All legs simple, unmodified, ending in a stalked
sucker _Sarcoptidæ_.

Front legs short, enlarged, and modified for
clasping; all legs end in one or two stout claws _Cheyletidæ_.

IXODIDÆ.

The Ixodidæ, or ticks, are rather large, flat, leathery-skinned mites, which suck the blood of various animals. In the male the dorsum of the body is nearly covered by a corneous shield, while in the female this shield occupies only the anterior part of the body. In the female the body swells to enormous proportions as she engorges herself on the blood of the host. At the posterior margin of the body there are in many forms a series of lobes or festoons. There is no species of tick that is commonly found on rats, but four species that normally infest other animals have been taken from them.

FIG. 8.—Mite (_Lælaps echidninus_).
]

1. On the venter is a groove in front of the anus and
extending back each side; no festoons to _Ixodes
posterior margin of body; palpi rather long ricinus_.

On the venter there is no groove in front of the
anus, but usually one behind; festoons distinct
in males and unengorged females 2

2. Palpi very short with transverse ridges; shield of
female narrowed behind eyes; stigmal plate nearly _Margaropus
round annulatus_.

Palpi short, without transverse ridges; shield of
female not narrowed behind eyes; stigmal plate _Rhipicephalus
comma shaped sanguineus_.

Palpi elongate, without ridges; shield of female _Hyalomma
broad; stigmal plate oval, aegypticum_.

_Ixodes ricinus_ Linné.

The shield of the female is elliptical, plainly longer than broad, sides not suddenly narrowed behind, and there is no eye-spot at each lateral corner. The coxa I has a long sharp spine.

This is a common European tick found on sheep, cattle, dogs, etc., and it has been taken a few times in this country. Neumann has recorded its capture from _Mus decumans_.

_Margaropus annulatus_ Say.

The shield is plainly longer than broad, with a distinct eye-spot at each lateral corner, and behind the eye the shield is suddenly narrowed; the coxæ of the female are without spines, but the male has 2 on coxæ I. This is the common cattle tick of the United States, and disseminates the Texas fever. Mr. Hunter has taken it once from a rat in a barn at Dallas, Tex.

_Rhipicephalus sanguineus_ Latreille.

The shield of the female is oval, and longer than broad, with an eye-spot at each outer corner. Coxa I with 2 teeth; a smaller tooth on each of the other coxæ. Stigmal plate long, comma shaped. In the male there is a corneous plate each side of the anus, and on middle of posterior margin a projection, or short tail.

This species is common in tropical countries, and Nuttall has recorded specimens from the black rat in India.

_Hyalomma aegypticum_ Linné.

The shield of the female is as broad as long, and the eye-spot is slightly above each outer corner. Coxa I has 2 large teeth, and a small tooth on each of the other coxæ. In the male there are 2 corneous plates each side of anus, and behind is a pair of small tubercles.

This is a common tick in the warmer parts of the Old World; and Nuttall has recorded young specimens on the black rat.

GAMASIDÆ.

The Gamasid mites, although much smaller than the ticks, are large enough to be seen by the naked eye. They are active, and most are not parasitic, at least for part of their time. The palpi are simple, of 5 joints; the mandibles are elongate, retractile, and usually chelate at tip. There are no eyes. The dorsum and often the venter shows one or more corneous shields or plates, frequently a number of them; one or two on the dorsum, and on the venter one between the coxæ, called the sternal plate; one behind this, the genital plate; one behind the latter, the ventral plate; and one surrounding the anus, the anal plate. Frequently some of these are absent or united to one of the others.

The legs are slender, usually of 6 joints, with a long tarsus that terminates in 2 claws, and often a sucker, or caroncle. The stigmata, or spiracles, are lateral above and between coxæ II and IV, and usually provided with a slender peritreme reaching forward toward the head. Nearly all the Gamasidæ deposit eggs, and the young often differ considerably from the adult in structure. There are two, and perhaps sometimes three, nymphal stages. In one of these nymphal stages the mite is apt to attach itself to an insect for the purpose of being carried to a similar locality, where it may feed and mature. The coprophagous and xylophagous insects are especially concerned in the diffusion of these mites.

There are, however, quite a number of species that are genuine parasites of insects and other animals. Those occurring on rats belong to two genera, Myonyssus and Lælaps. They can be separated as follows:

1. Anal plate small, much smaller than the ventral
plate _Lælaps_.

Anal plate large, larger than the ventral plate _Myonyssus_.

_Myonyssus._—This genus is made by Tiraboschi for one species: _Myonyssus decumani_ Tiraboschi.

Body oval; legs short and stout, all tarsi with a large caroncle with two short claws; coxæ II have a large tooth on the anterior border, none of coxæ with spines; sternal plate much broader than long, with three bristles each side; ventro-genital plate much longer than broad, broadest behind, bordered with bristles; anal plate very large, nearly one and a half times as broad as long; three large spines each side on venter. Length, 0.95 millimeter.

Found in Italy on _Mus decumans_.

_Lælaps._—This genus embraces a large number of species, several of which occur on small animals, such as the muskrat, ground hog, and chipmunk. Three have been recorded from rats, one of these from California. The dorsal plate is covered with hairs or bristles, and there are usually stout bristles on the margins of the plates on the venter. There is also a bristle, or a spine, at the tip of the anal plate. The legs are short and stout, with a distinct caroncle, and two claws.

1. Dorsum with numerous fine hairs; no stout spines on
coxæ _L. stabularis_.

Dorsum with fewer, but stouter spine-like bristles;
each coxa has a stout spine 2

2. Body but little longer than broad; ventral plate
longer than broad _L. agilis_.

Body much longer than broad; ventral plate about as
broad as long _L. echidninus_.

_Lælaps echidninus_ Berlese.

Dorsum of body almost wholly covered with a shield, with rows (six in front, eight behind) of stout, curved bristles, a longer pair near front margin, and some around lateral and posterior margins. Legs short and stout, tarsi about twice as long as preceding joint; each coxa bears a stout spine near middle. Palpi very short; sternum with three stout bristles or spines each side; ventral plate with four stout bristles each side; anal plate with a stout apical bristle, and a small one each side. Length, 1 millimeter.

Occurs commonly on rats in warm countries, and known from California. It may possibly aid in the transmission of disease.

_Lælaps agilis_ Koch.

Similar in many respects to _L. echidninus_ but differ in the shorter and proportionately broader body, barely longer than broad, and in the weaker and shorter spines on dorsum and on the ventral plates; there are also some small short spines on the general surface of the venter. Length, 0.7 millimeter.

Recorded from rats from Europe and Africa.

_Lælaps stabularis_ Koch.

The body is of the same general shape as in _L. echidninus_, but the dorsum is clothed with 12 to 18 rows of fine short hairs. The first pair of legs is more slender than in the other species, and the hind legs are also more elongate; the coxæ do not have the stout spines seen in the other species, and the bristles on the sternal and ventral plates are much less stout; the general surface of the venter has many hairs, the anal plate has a short apical bristle. Length, 1.2 millimeters.

Taken on the brown rat in Italy; also found in manure.

CHEYLETIDÆ.

This family consists of small, soft-bodied mites, that are parasitic or predaceous in habits. The palpi are small, three or four jointed; the mandibles are styliform and retractile; and the breathing spiracles open near the mouth parts. The species that occur on rats belong to the genus _Myobia_.

_Myobia._—The body is elongate, fully twice as long as broad, tipped by a pair of long, stout bristles. The first pair of legs is enlarged and shortened, with a terminal hook to grasp hairs; the other legs are short, simple, and far apart. The palpi and mouth parts are very small, and the dorsum bears stout bristles. They are supposed to feed on the exudations of the skin, but it would seem more probable, from the nature of the mandibles, that they pierced the skin to secure food. All are very small, not one-half a millimeter long. Of the several species two have been recorded from rats.

1. Dorsum of female with spines all acute and sharp _M. musculi_.

Dorsum of female with some of the posterior spines
flattened and rather scale like _M. ensifera_.

_Myobia musculi_ Schrank.

This occurs on various mice and moles, and once recorded from the brown rat. It has been taken in this country on mice. It lives at the base of the hairs.

_Myobia ensifera_ Poppe.

This was described from the brown rat in Europe. The female is separated from _M. musculi_ by having about six of the posterior dorsal spines flattened and scale like; in the male the six dorsal spines are longer, and the small spines much smaller than in _M. musculi_.

SARCOPTIDÆ.

These are the itch and scab mites. The body is soft, rounded, and whitish in color. The legs are very short, of five joints, and end in one or two claws, and often a pediceled sucker. The palpi are small and short, of three joints, but the basal is usually united to the rostrum. There are no spiracles, and respiration is therefore through the general surface of the skin. The sexes are often quite different in structure. The females usually deposit eggs, the larvæ are hexopod, and there are two nymphal stages. They are all parasites, mostly on mammals and birds, and often burrow in the skin, causing mange, or scabies.

Only one species has been taken on rats; this belongs to the genus _Notoedres_.

_Notoedres._—In this genus the third pair of legs of the male and the third and fourth of the female have no sucker at the tip. The anal opening is on the posterior part of the dorsum. The three known species are parasitic on mammals—one on the cat, one on the rabbit, and the third on rats.

_Notoedres muris_ Mègnin.

This is a rounded mite, with finely striate skin, a small triangular rostrum; in front the four anterior legs project a little beyond the body, and each ends in a long pedicellate sucker; the third and fourth pairs of legs are not visible from above, and each ends in a long bristle. There are a few short hairs around the anal aperture and about ten others in front of these. The species measures about 0.3 to 0.4 millimeter long.

It usually occurs about the ears and the genital organs of the host, and has been taken from both the brown and black rat in Europe. The _Sarcoptes alepis_ Railliet and Lucet is the same species.

DEMODECIDÆ.

Besides the mites above described, a form of _Demodex_ has been recorded from rats, but the species is not given. These mites are very tiny, with elongate body, the posterior part annulate, the front part with eight very short legs. They inhabit the hair follicles of various mammals. That on the rat may have been only an accidental occurrence of some species normally on another animal.

THE INTERNAL PARASITES OF RATS AND MICE IN THEIR RELATION TO DISEASES OF
MAN.

By CH. WARDELL STILES, _Chief_, and CHARLES G. CRANE, B. S., _Assistant,
Division of Zoology, Hygienic Laboratory, United States Public Health
and Marine-Hospital Service_.

SUMMARY.

Rats and mice may harbor 11 species of internal parasites which come
into consideration as possible or established parasites of man. From
this point of view, 7 of the parasites are of more academic interest
than practical importance. The rat may, however, be viewed as the
practical, theoretical, and permanent reservoir for one zooparasitic
disease (trichinosis) of considerable importance, and of at least one,
perhaps two, other zooparasitic infections (“_Lamblia duodenalis_” and
_Hymenolepis diminuta_) of much less importance. Its possible future
rôle in connection with sleeping sickness should not be entirely
ignored.

From the standpoint of internal zooparasitism, therefore, the present
public health interest in rats and mice centers in trichinosis. This
disease will probably never be eradicated from man until rats and mice
are practically eradicated, and any rational public health campaign
directed against trichinosis must take the rat into serious
consideration.

The eradication of rats and mice would be a very substantial
contribution toward a reduction and eradication of trichinosis.

INTRODUCTION.

From the habits of rats, it is to be expected that they harbor many species of parasites, and on account of their presence in our houses the question naturally arises as to whether any of these parasites are transmissible, either directly or indirectly, to man.

The species of internal parasites which come especially into consideration in this connection are the following:

PROTOZOA: _Chlamydophrys enchelys_ (p. 88), _Lamblia duodenalis_ (p.
89), _Trypanosoma gambiense_ (p. 94).

TREMATODA: None.

CESTODA: _Cysticercus cellulosæ_ (p. 95), _C. fasciolaris_ (p. 96),
_C. pisiformis_ (p. 95), _H. murina_ Duj. [= _fraterna_] (p. 96),
_Hymenolepis diminuta_ (p. 98).

NEMATODA: _Trichinella spiralis_ (p. 101).

ACANTHOCEPHALA: _Gigantorhynchus moniliformis_ (p. 108).

ARACHNOIDEA: _Linguatula denticulata_ (p. 110).

Of these 11 species, the trichina worm (sometimes called the flesh worm) exceeds all the others combined, both in frequency and importance, as a cause of disease in man.

PROTOZOA.

Genus CHLAMYDOPHRYS[U] Cienkowski, 1876.

Footnote U:

SYNONYM.—_Leydenia_ Schaudinn, 1896.

Species CHLAMYDOPHRYS ENCHELYS[V] (Ehrenberg.)

Footnote V:

SYNONYMS.—_Difflugia enchelys_ Ehrenberg; _Chlamydophrys stercorea_
Cienkowski; _Leydenia gemmipara_ Schaudinn, 1896; _Chl. enchelys_
(Ehrenberg) Braun.

A very peculiar organism has been described under the name of _Leydenia gemmipara_ Schaudinn, 1896. This was found in fluid, obtained by puncture, from two ascites patients in Berlin, Germany. More recently Schaudinn has concluded that _Leydenia gemmipara_ represents an abnormal condition of a protozoon known as _Chlamydophrys_. The latter passes through the intestinal tract of various animals (as man, mice, squirrels, rabbits, cattle), and thus is occasionally found in fresh human stools. According to Schaudinn, if pathological conditions in the colon cause an alkaline reaction of its entire content, the usual shell formation in _Chlamydophrys_ fails to take place, the organisms then multiply in an atypical manner by division and budding, and the result is the structure described as _Leydenia gemmipara_.

Genus LAMBLIA[W] R. Blanchard, 1888.

Footnote W:

SYNONYMS.—_Dimorphus_ Grassi, 1879 (not Haller, 1878, arachnoid);
_Megastoma_ Grassi, 1881 (not de Blainville, mollusk; not Swains.,
1837, bird; not Costa, 1850, fish; not Megerle, mollusk); “_Dimorpha_
Grassi” of Senn, 1901 (not _Dimorpha_ Jur., 1807, hymenopteron; not
Gray, 1840, mollusk; not Hodgs., 1841, bird); _Megastroma_
Schneidemuehl, 1898, misprint.

GENERIC DIAGNOSIS.—_Polymastigidæ_: Body bilaterally symmetrical,
pyriform, excavate antero-ventrally to form a sucker; flagella
directed posteriorly; 3 pairs inserted on margin of the sucker, 1 pair
at posterior end of body. Parasitic in intestine of mammals.

TYPE SPECIES.—_Lamblia duodenalis_ s. l. (“_L. intestinalis_” of man).

Flagellate protozoa belonging to this genus are reported as parasitic in the intestinal canal of various species of mammals. At present the forms in question are usually looked upon as belonging to the species _L. duodenalis_. Evidence is, however, accumulating (p. 92) to the effect that there are at least three distinct species of _Lamblia_ (“_L. intestinalis_” of man, _L. muris_ of mice, and _L. cuniculi_ (or _duodenalis_?) of rabbits). Admitting that there may be three species, the intertransmissibility of these forms from one host to another remains to be investigated to some extent. It seems thus far definitely proved that the form which occurs in man is transmissible to mice, rabbits, and guinea pigs, hence mice still remain a source of danger in respect to the infection in man. To exactly what extent this fact is of academic interest or of practical significance is at present _sub judice_.

Species LAMBLIA DUODENALIS[X] (Davaine, 1875) Stiles, 1902, s. l.

Footnote X:

SYNONYMS.—_Cercomonas intestinalis_ Lambl, 1859, in man (not _Bodo_
(_Cercomonas_) _intestinalis_ (Ehrenberg, 1838) Diesing, 1850, in
frogs; not _Cercomonas intestinalis_ (Ehrenberg, 1838) Perty, 1852);
_Hexamita duodenalis_ Davaine, 1875, in rabbits; _Dimorphus muris_
Grassi, 1879, in _Mus_; _Megastoma entericum_ Grassi, 1881 (=
_Dimorphus muris_ renamed); _Megastoma intestinale_ (Lambl, 1859) R.
Blanchard, 1885; _Lamblia intestinalis_ (Lambl, 1859) R. Blanchard,
1888; “_Megastoma intestinalis_” of Leclerq, 1890; “_Cercomonas
intistinalis_ Lambl” of L. Pfeiffer; “_Megastroma entericum_ Grassi,
1881” of Schneidemuehl, 1898; “_Dimorpha muris_ Grassi” of Senn, 1900.

[Figs. 9 to 15.]

SPECIFIC DIAGNOSIS.—_Lamblia_ (p. 88): Body pyriform, 5 to 16μ (21μ
Lambl) long, 4 to 12.5μ (8.6 to 11μ Lambl) broad; flagella 9 to 14μ
long; anterior end bluntly rounded, posterior end sharply pointed,
dorsum convex, antero-ventrally concave, venter flat to convex;
antero-ventral concavity forms a sucker, the margins of which project
from the surface and are contractile. Four pairs of ventral
posteriorly directed flagella, arranged as follows: 1 pair insert on
anterior margin of sucker; 2 pairs on posterior margin of sucker, near
median line; 1 pair on posterior extremity. Body membrane (“cuticula”)
very delicate, permitting some change of body form; protoplasm finely
granular; nucleus dumb-bell shaped, pre-equatorial. Vacuoles not
observed. Copulation sucker-to-sucker, followed by an encystation, in
which stage complicated nuclear changes occur; cysts 10 by 7μ.

HABITAT.—Upper portion of small intestine of man (_Homo_); also of the
common house mouse (_Mus musculus_), the brown rat (_M. decumanus_),
the black rat (_M. rattus_), “_Mus sylvestris_” [=? _M. decumanus_],
field mouse (_Microtus arvalis_), water mole (_Arvicola amphibius_),
rabbits, guinea pigs, domesticated cats, dogs, and sheep.

GEOGRAPHICAL DISTRIBUTION.—Europe, Egypt, and United States.

This parasite is very common in animals in certain parts of Europe, and cases of its presence in man have been reported by a number of authors (Lambl, 1859; Grassi, 1881; Perroncito, 1888; Moritz, 1891; Moritz & Holzl, 1892; Roos, 1893; Kruse & Pasquale, 1894; Piccardi, 1895; Sievers; Mueller; Frshezjesski & Ucke; Stiles, 1902; Braun, 1908; etc.). The indications are that it is more common in man than is generally assumed.

Possibly man becomes infected through eating food (as bread, etc.) which has been soiled by the excrements (containing the encysted stage) of mice and rats. Grassi infected himself, Perroncito infected mice and rabbits, and Stiles infected guinea pigs by feeding to them human feces containing the encysted stage.

The parasite may be present in large numbers. Moritz estimated a discharge of 18 milliards within twenty-four hours from one of his patients. It has been observed in healthy persons and also in cases of various diseases, but especially in children and in cases of tuberculosis. It is an inhabitant chiefly of the duodenum and jejunum, where it attaches itself (fig. 13) by means of the sucker to the epithelial cells. It is rarer in the ileum. In case the stomach is alkaline (carcinoma) the parasite may occur in this organ (Cohnheim, Zabel). In P. Schmidt’s case the hydrochloric acid was 1 per cent. In case the intestinal peristalsis is normal the parasite becomes encysted in the colon, so that usually only the encysted stage is found in the feces; but in case of increased peristalsis and diarrhea the organisms have not time to encyst, so that the free stages are observed in the stools. As the parasites become cool motion decreases; when raised to high temperature, as 50° C., motion becomes slow, and the organisms die at 52° C. or below 0° C.

FIG. 9.—Lateral view of encysted _Lamblia duodenalis_.

FIG. 10.—Cyst from large intestine.

FIG. 11.—Ventral view of _Lamblia_.

FIG. 12.—Lateral view.

FIG. 13.—Epithelial cells of the villous coating of the small
intestine infested with _Lamblia_. (After
Grassi & Schewiakoff, 1888, pl. 15, figs. 1, 2, 5, 11, 12.)
]

FIG. 14.—An epithelial cell with parasitic _Lamblia_. Greatly
enlarged. (After Grassi & Schewiakoff, 1888, pl. 15, fig. 6.)
]

FIG. 15.—An individual in the act of joining an epithelial cell.
(After Grassi & Schewiakoff, 1888, pl. 15, fig. 7.)
]

PATHOGENICITY.—Opinion differs as to the pathogenicity of this organism. Perroncito (1902b) reports it as causing a fatal disease in rabbits. Braun (1908) is inclined to consider it harmless. From conversation with Doctor Hemmeter, we are persuaded that in his case in a child in Baltimore the parasite was not without effect. Possibly the question as to its pathogenicity is a relative one in that light infections may produce no recognizable disturbance, while heavy infections may produce recognizable effects. Doctor Hemmeter’s original letter regarding his case contained the following notes:

Patient, male, white child, 3 years old, born in Maryland. Had
recurrent attacks of colitis all its life; three acute attacks within
the last three weeks, accompanied by fever, distended abdomen,
sensitive, etc. Stools have always been like putty, containing large
amount of mucus, and some blood streaks; fever lasting three days, no
pronounced diarrhea, 2 to 3 passages per day; intervals between
attacks variable, stools at such times like putty, also with mucus.

Later information from Doctor Hemmeter states that the disappearance of the parasites from the stools coincided with improvement in the child’s condition.

CLINICAL DIAGNOSIS.—The fresh unstained stools should be examined microscopically; or the diluted stool may be stained with methylene blue, by which nearly all objects become promptly stained, except _Lamblia_, which remains grayish white (Roos, 1893) for several hours.

TREATMENT.—Attempts to expel _Lamblia_ have not always met with marked success. Among the drugs used are male fern, sulphate of quinine, naphthol, calomel, hydrochloric acid, and arsenic. Grassi appears to have had success with calcined magnesia.

FIG. 16.—“_Lamblia intestinalis_” of man. (After Bensen, 1908, fig.
5.)
]

FIG. 17.—Copulation cyst of “_Lamblia intestinalis_” of man. (After
Bensen, 1908, fig. 5.)
]

THE DIVISION OF _Lamblia duodenalis_ INTO SEPARATE SPECIES.—Bensen (1908) has recently divided _Lamblia duodenalis_ s. l. into three species: _L. “intestinalis,”_ _L. muris_, and _L. cuniculi_. His preliminary paper seems to offer fairly convincing data for the correctness of his interpretation, but it may be well to await the publication of his more complete paper, in which he promises fuller details, before the species are definitely accepted. Several nomenclatural points will come up for consideration in this connection.

_Lamblia intestinalis_, which Bensen accepts as name for the species (fig. 16) occurring in man, can not be accepted, as this name is based on _Cercomonas intestinalis_ Lambl, 1859, which is invalidated by _Cercomonas intestinalis_ (Ehrenberg, 1838) Perty, 1852, found in frogs.

_Lamblia muris_ (fig. 18) will probably stand, based on _Dimorphus muris_ Grassi, 1879.

_Lamblia cuniculi._—There is some doubt as to the status of this name. Davaine (1875a, 128–129) has described from rabbits a protozoon, which he designated as _Hexamita duodenalis_ and which Railliet (1893a, 169) identifies as a synonym of _Lamblia intestinalis_ (Lambl). On basis of the principle that identifications are to be accepted as correct until shown to be incorrect, Stiles has accepted _duodenalis_ as name (1902) for the form in question. The question now arises as to the relation of _cuniculi_ to _duodenalis_. If they are accepted as identical, _duodenalis_ will supplant _cuniculi_, and a new name must be given to the form found in man. If _duodenalis_ is taken as identical with _intestinalis_ Lambl, _duodenalis_ remains as name for the form in man, and _cuniculi_ Bensen will stand for the species in rabbits. If Railliet’s interpretation of synonymy be shown to be incorrect by proving that _duodenalis_ Davaine is not to be considered in connection with either form, a new name must be given to _intestinalis_ Lambl.

FIG. 18.—_Lamblia muris_ of mice. (After Bensen, 1908, fig. 1.)
]

FIG. 19.—Autogametocyte of _Lamblia muris_. (After Bensen, 1908, fig.
6.)
]

Genus TRYPANOSOMA s. l.

An extensive group of parasitic protozoa, known as “trypanosomes,” has recently been the basis of considerable literature. The genus _Trypanosoma_ was originally based upon a species (_T. rotatorium_) found in frogs, and while most trypanosomes have been described as members of this genus several authors have separated out certain forms into separate genera.

Luehe (1906) has recently placed the trypanosomes of mammals in the

Genus TRYPANOZOON Luehe, 1906.

One of these species (_Trypanozoon gambiense_, usually known as _Trypanosoma gambiense_) is the cause of “sleeping sickness” in man, and has been transmitted in laboratory experiments to rats and mice.

FIG. 20.—An isolated pork-measle bladder worm (_Cysticercus
cellulosæ_), with extended head. Greatly enlarged. (After Stiles,
1898a, 90, fig. 76.)
]

Just what practical importance there may be in the ability of the parasite to live in rats and mice remains to be seen, but theoretically this biological factor may possibly become one of considerable magnitude. At present the least conclusion to be drawn is that it adds one more to the many arguments in favor of a world-wide destruction of rats and mice.

Several trypanosomes, other than _Trypanozoon gambiense_, are transmissible by experiment to rats and mice, while one species (_Trypanozoon lewisi_) has rats for its normal host, and two other species (_Trypanozoon duttoni_ and _Trypanosoma musculi_ Kendall, 1896) are reported originally from the mouse.

CESTODA—TAPEWORMS.

Of the five cestodes mentioned as coming into consideration in the subject under discussion, only one (_Hymenolepis diminuta_) need be considered seriously.

CYSTICERCUS CELLULOSÆ—TÆNIA SOLIUM.

[Fig. 20.]

The larval cestode known as _Cysticercus cellulosæ_ (which causes “measles” in swine) develops (when eaten by man) into a tapeworm which is known as _Tænia solium_. This larva is also reported as encysted in the peritoneum of _Mus rattus_.

FIG. 21.—Portion of mesentery of rabbit infected with _Cysticercus
pisiformis_. Natural size. (After Railliet, 1893a, 216, fig. 114.)
]

Even if it be granted that the specific determination of the specimen in question as _Cysticercus cellulosæ_ is correct, the occasional infection of rats with this parasite would be of very little practical significance in this country from a public health point of view, as we do not use rats for food for man. Theoretically it is possible to conceive of combinations of circumstances in which such infection in the rat might under certain conditions eventually affect man, but the chances are so remote as to be negligible, especially when compared with the much greater questions which demand attention.

CYSTICERCUS PISIFORMIS—TÆNIA PISIFORMIS.

[Fig. 21.]

The larval stage of this parasite occurs in rabbits, the adult stage in canines. Parona (1901) reports the occurrence of the larval stage in _Mus brasiliensis_, and Vital has recorded the presence of the adult stage in man.

In view of the fact that Galli-Valerio was unable to infect himself experimentally with this species, the specific determination made by Vital is open to some question. Even assuming that this tapeworm may develop in man, the presence of the larval stage in rats is of such little importance as to be negligible.

CYSTICERCUS FASCIOLARIS—TÆNIA TENIÆFORMIS.

[Fig. 22.]

This encysted larval tapeworm is exceedingly common in the liver of rats and mice, and when swallowed by cats it develops into an adult tapeworm.

FIG. 22.—Larval stage of _Tænia teniæformis_. Natural size. (After
Leuckart, 1880, 450, fig. 202.)
]

There are two possible points of view in connection with which this parasite is of indirect interest in public health matters: (1) Occasionally these encysted parasites are mistaken for lesions of tuberculosis; (2) Krabbe (1880) relates that in Jutland there exists a folk custom of eating chopped raw mice in case of retention of urine, and in this connection the point has been raised that the possibility is not excluded that such action might eventually give rise to infection of man by the parasite in question. No case of such infection in man is as yet established.

HYMENOLEPIS MURINA[Y] (Dujardin, 1845) = HYMENOLEPIS NANA FRATERNA[Y]
Stiles, 1906.

Footnote Y:

SYNONYM.—_Tænia murina_ Dujardin, 1845 (not Gmelin, 1790).

[Figs. 23 and 24.]

Under the name _Tænia murina_, Dujardin (1845) described for rats a tapeworm which has been identified by a number of authors (including Stiles) as identical with the dwarf tapeworm (_Hymenolepis nana_) of man. If this identification be correct, the rats must be considered as the great disseminators of this tapeworm. Serious doubts have been raised, however, as to whether the tapeworm in man is not in reality distinct from that of the rat, and the evidence in favor of such conclusion is accumulating. Some slight differences between the two forms have been noticed, but by some authors these differences have been considered insufficient to hold the two worms apart. Looss has tried to infect rats with the dwarf tapeworm found in man in Egypt, but his results have been negative. Here in Washington Stiles has repeatedly attempted to infect rats with the dwarf tapeworm found in man in the United States, but thus far no positive infection has occurred in the rodents. In Italy, Grassi attempted to transmit the rat form to six persons, and in one case he found tapeworms, but in view of the frequency of _H. nana_ in Italy the significance of this one instance has been questioned; Grassi was not successful in trying to infect rats with _H. nana_ of man.

FIG. 23.—Longitudinal section of the intestinal villus of a rat,
containing cystic stage of dwarf tapeworm. Enlarged. (After Grassi &
Rovelli, 1892a, pl. 3, fig. 25.)
]

FIG. 24.—Adult dwarf tapeworm (_Hymenolepis nana_) of man. Enlarged.
(After Leuckart, 1863, p. 393, fig. 112.)
]

Thus at present the evidence is to the effect that rats and mice are not to be viewed as the source or reservoir for the dwarf tapeworm (_H. nana_) of man.

HYMENOLEPIS DIMINUTA[Z] (Rudolphi, 1819) R. Blanchard, 1891.

Footnote Z:

SYNONYMS.—_Tænia diminuta_ Rudolphi, 1819; _T. leptocephala_ Creplin,
1825; _Hymenolepis flavopunctata_ Weinland, 1858; _Tænia (Hymenolepis)
flavopunctata_ Weinland, 1859; _H. (Lepidotrias) flavopunctata_
Weinland, 1861; _T. flavomaculata_ Leuckart, 1863; _T. “flavopuncta”_
Cobbold, 1864 (misprint); _T. “flaviopunctata”_ Vogt, 1878 (misprint);
_T. “flavopunktata”_ Stein, 1882; _T. varesina_ E. Parona, 1884; _T.
minima_ Grassi, 1886; _T. “septocephala”_ Perroncito and Airoldi, 1888
(misprint); _Hymenolepis diminuta_ (Rudolphi, 1819) Blanchard, 1891;
_“Hymenolepsis” flavopunctata_ of Osler, 1895, and other authors
(misprint); _T. “varerina”_ Huber, 1896 (misprint for _T. varesina_);
_T. “flavapunctata”_ Simon, 1896 (misprint); _T. “leptocefala”_
Previtera, 1900; _T. “ceptocephala”_ Lussana and Romaro [? date]
(misprint); _Tenia flavopunctata_ (Weinland, 1858) Packard, 1900.

[Figs. 25 to 30.]

FIG. 25.—Strobila of _Hymenolepis diminuta_. Natural size. (After
Grassi, 1881, pl. 11, fig. 1.)
]

FIG. 26.—Head and anterior portion of _H. diminuta_ from the rat.
Enlarged. (After Zschokke, 1889, pl. 1, fig. 21.)
]

SPECIFIC DIAGNOSIS.—_Hymenolepis_: Strobila 10 to 60 millimeters in
length, 2.5 to 4 millimeters in maximum breadth; composed of 800 to
1,300 segments. Head small, almost globular; 200 to 600μ in width;
rostellum rudimentary, pyriform, only slightly protractile; hooks
absent; suckers globular, near the apical portion of the head, 80 to
160μ in diameter. Neck usually short. Segments throughout strobila
broader than long. Genital pores on left margin, near the junction of
the anterior and middle thirds of each segment. Three testes in each
segment; vas deferens dilates into a prominent seminal vesicle before
entering the cirrus pouch, within which also is a vesicle. Gravid
uterus occupies most of the proglottids; its cavity is subdivided into
a large number of incompletely separated compartments filled with
eggs. Eggs round or slightly oval; outer membrane 54 to 86μ in
diameter, yellowish in color, may be radially striated; inner membrane
24 by 20μ to 40 by 35μ in diameter, with mammillate projection at each
pole often not apparent; between outer and inner membranes a prominent
third layer of albuminous substance, often appearing as two delicate
smooth membranes, with intervening space filled by a granular
coagulum; embryonal hooks 11 to 16μ in length.

FIG. 27.—Male and female organs of _H. diminuta_: _c. p._, cirrus
pouch; _g. p._, genital pore; _ov._, ovary; _rec. sem._,
receptaculum seminis; _s. g._, shell gland; _t._, testiculæ; _ut._,
uterus; _vag._, vagina; _v. def._, vas deferens; _v. ef._, vas
efferens; _ves. sem._, vesicula seminalis; _y. g._, yolk gland.
Enlarged. (After Zschokke, 1889, pl. 2, fig. 22.)
]

FIG. 28.—Gravid segment of _H. diminuta_. Enlarged. (After Grassi,
1881, pl. 11, fig. 15.)
]

FIG. 29.—Egg of _H. diminuta_ from man. Greatly enlarged. (After
Bizzozero, 1889a, pl. 4, fig. g″.)
]

HABITAT.—Adults in small intestine of brown or Norway rat (_Mus
decumanus_), black rat (_M. rattus_), house mouse (_M. musculus_),
Egyptian or roof rat (_M. rattus alexandrinus_), wood or field mouse
(_M. sylvaticus_), _Rhipidomys pyrrhorhinus_ [according to Linstow,
1878a, 23], and man (_Homo sapiens_).

DEVELOPMENT.—The larval stage (_Cercocystis H. diminutæ_) occurs in
larval and adult meal moths (_Asopia farinalis_); in young and adult
earwigs (_Anisolabis annulipes_); and in adult beetles (_Acis spinosa_
and _Scaurus striatus_).

GEOGRAPHIC DISTRIBUTION.—Massachusetts, Pennsylvania, Nebraska, Iowa,
District of Columbia, Maryland, Brazil, Italy, Germany, France,
Austria.

This parasite is certainly more common in man in this country than has heretofore been assumed, but fortunately it seems to be one of the most harmless and most easily expelled tapeworms occurring in man.