In this cla.s.s the antennae are apparently wanting, wings are never present, and the adults are usually provided with four pairs of legs.
Scorpions, harvest-men, spiders, mites, etc.
HEXAPODA (Insects)
True insects have a single pair of antennae, which is rarely vestigial, and usually one or two pairs of wings in the adult stage. Familiar examples are c.o.c.kroaches, crickets, gra.s.shoppers, bugs, dragon-flies, b.u.t.terflies, moths, mosquitoes, flies, beetles, ants, bees and wasps.
ORDERS OF THE ARACHNIDA
a. Abdomen distinctly segmented. A group of orders including scorpions, (fig. 11), whip-scorpions (fig. 10), pseudo-scorpions, solpugids (fig. 12) harvest-men (daddy-long-legs or harvestmen), etc.
ARTHROGASTRA
aa. Abdomen unsegmented, though sometimes with numerous annulations SPHaeROGASTRA
b. A constriction between cephalothorax and abdomen (fig. 7). True Spiders ARANEIDA
bb. No deep constriction between these parts.
c. Legs usually well developed, body more or less depressed (fig.
49). Mites ACARINA
cc. Legs stumpy or absent, body more or less elongate or vermiform, or if shorter, the species is aquatic or semi-aquatic in habit.
d. Four pairs of short legs; species inhabiting moss or water.
Water-bears. TARDIGRADA
dd. Two pairs of clasping organs near the mouth, instead of legs, in the adult; worm-like creatures parasitic within the nasal pa.s.sages, lungs, etc. of mammals and reptiles (fig. 148).
Tongue worms. LINGUATULINA
[Ill.u.s.tration: 148. Linguatula. (_a_) larva; (enlarged). (_b_) adult; (natural size).]
ACARINA[E]
a. Abdomen annulate, elongate; very minute forms, often with but four legs (fig. 62). DEMODICOIDEA
b. With but four legs of five segments each. Living on plants, often forming galls. ERIOPHYIDae
bb. With eight legs, of three segments each. Living in the skin of mammals. DEMODICIDae
To this family belongs the genus DEMODEX found in the sebaceous glands and hair follicles of various mammals, including man. _D.
phylloides_ Csokor has been found in Canada on swine, causing white tubercles on the skin. _D. bovis_ Stiles has been reported from the United States on cattle, upon the skin of which they form swellings. D. FOLLICULORUM Simon is the species found on man. See page 78.
aa. Abdomen not annulate nor prolonged behind; eight legs in the adult stage.
b. With a distinct spiracle upon a stigmal plate on each side of the body (usually ventral) above the third or fourth c.o.xae or a little behind (fig. 50); palpi free; skin often coriaceous or leathery; tarsi often with a sucker.
c. Hypostome large (fig. 50), furnished below with many recurved teeth; venter with furrows, skin leathery; large forms, usually parasitic. IXODOIDEA
d. Without scutum but covered by a more or less uniform leathery integument; festoons absent; c.o.xae unarmed, tarsi without ventral spurs; pulvilli absent or vestigial in the adults; palpi cylindrical; s.e.xual dimorphism slight. ARGASIDae
e. Body flattened, oval or rounded, with a distinct flattened margin differing in structure from the general integument; this margin gives the body a sharp edge which is not entirely obliterated even when the tick is full fed.
Capitulum (in adults and nymphs) entirely invisible dorsally, distant in the adult by about its own length from the anterior border. Eyes absent. ARGUS Latr.
f. Body oblong; margin with quadrangular cells; anterior tibiae and metatarsi each about three times as long as broad. On poultry, southwest United States. A. PERSICUS MINIATUS
_A. brevipes_ Banks, a species with proportionately shorter legs has been recorded from Arizona.
ff. With another combination of characters. About six other species of _Argas_ from various parts of the world, parasitic on birds and mammals.
ee. Body flattened when unfed, but usually becoming very convex on distention; anterior end more or less pointed and hoodlike; margin thick and not clearly defined, similar in structure to the rest of the integument and generally disappearing on distention; capitulum subterminal, its anterior portions often visible dorsally in the adult; eyes present in some species.
f. Integument pitted, without rounded tubercles; body provided with many short stiff bristles; eyes absent. On horses, cattle and man (fig. 48). OTIOBIUS Banks.
O. MEGNINI, a widely distributed species, is the type of this genus.
ff. Integument with rounded tubercles or granules; body without stiff bristles. ORNITHODOROS Koch.
g. Two pairs of eyes; tarsi IV with a prominent subterminal spur above; leg I strongly roughened. On cattle and man.
O. CORIACEUS
gg. No eyes; no such spur on the hind tarsi.
h. Tarsi I without humps above. _O. talaje._
hh. Tarsi I with humps above.
i. Tarsi IV without distinct humps above. On hogs, cattle and man. O. TURICATA
ii. Tarsi IV with humps nearly equidistant (fig. 142).
Africa. O. MOUBATA
[Ill.u.s.tration: 149. Haemaphysalis wellingtoni. Note short palpi. After Nuttall and Warburton.]
dd. With scutum or shield (fig. 50); festoons usually present; c.o.xae usually armed with spurs, tarsi generally with one or two ventral spurs; pulvilli present in the adults; s.e.xual dimorphism p.r.o.nounced. IXODIDae
e. With a.n.a.l grooves surrounding a.n.u.s in front; inornate; without eyes; no posterior marginal festoons; venter of the male with non-salient plates. Numerous species, 14 from the United States, among them I. RICINUS (fig. 49 and 50), SCAPULARIS, COOKEI, _hexagonus_, _bicornis_. IXODES Latr.
(including Ceratixodes).
ee. With a.n.a.l groove contouring a.n.u.s behind, or groove faint or obsolete.
f. With short palpi (fig. 149).
g. Without eyes, inornate, with posterior marginal festoons; male without ventral plates. Numerous species. _H.
chordeilis_ and _leporis-pal.u.s.tris_ from the United States. _Haemaphysalis_ Koch.
[Ill.u.s.tration: 150. Stigmal plate of Dermacentor andersoni; (_a_) of male, (_b_) of female. After Stiles. (_c_) Dermacentor variabilis, male; (_d_) Glyciphagus obesus; (_e_) Otodectes cynotis; (_f_) Tyroglyphus lintneri; (_g_) Tarsonemus pallidus; (_h_) a.n.a.l plate and mandible of Liponyssus; (_c_) to (_h_) after Banks.]