NO. BITING AND SUCKING INSECTS SNODGRASS 91 particularly in warmer regions ; it occurs in most of the coastal

ports of the United States,

and

has been

found

in a

number

of inland States

where

formerly the climate

was

supposed to be too severe for it (see

Ewing and

Fox, 1943).

Plague is probably

endemic

in

many

localities

among

wild rodents, but

human

infection

from

such sources is rare. In a study of plague

Fig.35.

—

^Thealimentary canal ofaflea, anditsstoppage with plaguebacteria.

(A-C, from Faasch, 1935; D, E, from Bacot and Martin, 1914.)

A, outline of a flea and the alimentary canal, left side. B, section of pro- ventriculus and anterior end of ventriculus, showing proventricular spines and stomodaeal valve. C, spines of proventriculus, with plague bacteria, Pastcurella pcstis,lodged amongthem. D, lengthwise section of oesophagus, proventriculus and anterior end of ventriculus from a rat flea, Nosopsyllus jasciatus (Bosc), with stomach entrance blocked by a plug of plague bacteria. E, a flea having the proventriculus and stomach entirely filled with mass of plague bacteria.

AInt, anterior intestine; An, anus; b, blood in oesophagus; Mai, Malpighian tubules; Oe,oesophagus;Pp,massofPastcurella pcstisin proventriculus;Prvnt, proventriculus; Rect, rectum; rp, rectal papillae; s, proventricular spines

;

SVlv, stomodaeal valve; Vent, ventriculus (stomach).

among

California

ground

squirrels

Eskey

₍₁₉₃₈₎ points out that the native fleas of these rodents are relatively inefficient vectors of the disease as

compared

with the fleas of domestic rats,

and do

not readily transmit plague

when

feeding, even

though

they themselves

may

be infected.

Eskey

suggests that the biting

and

swallowing of fleas is probably an important

means

of spreading the disease

among

animals.

VII.

THE THRIPS. ORDER THYSANOPTERA

The

thrips are small or minute piercing-and-sucking insects,

which

derive their ordinal

name from

the fact that their slender

wings

are

92

SMITHSONIAN MISCELLANEOUS COLLECTIONS

VOL. IO4 fringed with long hairs.

Some

species, however, have

no

wings,

and members

of the

same

species

may

be either

winged

or wingless.

The

thrips feed for the

most

part

on

the sap of plants, including fungi, but

some

attack other soft-bodied insects. Occasionally, also, they

become an annoyance

to

man,

particularly at places in the country or at bathing resorts

where

there happens to be a large infestation

on

neighboring host plants,

from which swarms

of the tiny insects

may

settle

on

exposedparts of the body, givingan unpleasant tickling feeling or a pricking sensation. Certain species, moreover, have been

found

not only to "bite"

human

subjects, but even to suck blood.

The

evidence against the thrips

on

this score is reviewed by Bailey (1936).

The Thysanoptera

are here introduced not because of their occasional role as

human

pests, but because the structure of their feeding organswill serve as an introduction to that in the next order, the Hemiptera, which, studied alone, is difficult to understand.

The head

of an ordinary thrips ("thrips" is singular

and

used also in the plural) is elongate with the face strongly receding

and

ending below in a small conical beak directed

downward

^(fig. 36

A). The

beak is

composed

of the

labrum

ⁱⁿ front

(Lm),

the maxillae

on

the sides,

and

the labium behind (Lb). Enclosed

by

these parts are a

median hypopharynx (Hphy), and

three stylets not

shown

in the figure.

The

space

between

the

hypopharynx and

the inner wall of the

labrum

is the food

meatus

(jm),

which

leads

up

into a strong sucking

pump (CbP)

with its dilator muscles ₍₅₎ arising

on

the extensive clypeal area of the head (Clp).

Between

the base of the

hypopharynx and

the labium the salivary duct opens into a

narrow

salivary pocket

(Sh). The

relations of the labrum, thehypopharynx, the cibarium (sucking

pump),

the salivary orifice,

and

the labium in the thrips are thus seen to be the

same

as in the cockroach (fig.

7

A). The

special features of the thrips are in the structure of the mandibles

and

maxillae, though all parts of the feeding organs are subject to irregularities of

form and an asymmetry

that

make

their study

somewhat

confusing.

The

mandibles of the thrips are not exposed externally; they are retracted into the

head

between the

labrum and

the maxilla

and

are contained in pouches of the

head

wall.

Only

the left mandible is

functionally developed, the right being either greatly reduced in size,

or absent.

The

persisting left mandible has the

form

of a stylet with

an

enlarged base ^(fig. 36 B),

and

is the piercing organ of the thrips.

According

to

Reyne

₍₁₉₂₇₎ the mandible has retractor but

no

pro- tractor muscles; in

some

forms, however, ^it is protractile

by

reason ofa leverlike connection with the

movable

labrum.

The

mandible is

no. 7 BITING

AND SUCKING

INSECTS

SNODGRASS

₉₃ said tobe used as a pick or

punch

operated by tapping

movements

of the head.

The

maxillae consist each of an external lobe

and

a mesal stylet (fig.

36

C).

The

outer lobe

(MxL),

as above noted, lies in the side of the conical beak; it presents a broad basal region (St), evidently the stipes, bearing a small

segmented

palpus (Pip)

and

a tapering terminal part (Ca),

which may

be taken for the galea.

The

slender

Fig. 36.

—

Feeding apparatus of a thrips. Order Thysanoptera. (A, B, C, from Peterson, 1915;

D

from Reyne, 1927.)

A, vertical section of head and beak of Hcrcinothrips femoralis (Reuter), showing sucking

pump

and its dilator muscles. B, mandible of same. C, maxilla of Frankliuiella tritici (Fitch). D, maxilla with muscles of the stylet of Heliothrips haemorrhoidalis (Bouche).

Br, brain; CbP, cibarial pump; Clp, clypeus; fm, food meatus^; Ga, perhaps the galea; Hphy, hypopharynx; Lb, labium; Lc, lacinia; Lm, labrum; Ivr, lacinial lever;

MdB,

base of mandible;

MdS,

mandibular stylet; mth, mouth

of cibarial pump;

MxL,

maxillary lobe;

MxS,

maxillary stylet (lacinia) ; Oe, oesophagus; Pip, palpus; SIDct, salivary duct; Slv, salivarium; sm, salivary meatus; SocG, suboesophageal ganglion; St, stipes.

Mitsclcs.

—

5, dilators of cibarial pump; 32, retractor of lacinia; 41, protractor of lacinia.

stylet

(MxS)

^{is in}

some forms

connected with the base of the stipes

by

^{a lever}

arm

(Ivr),

and

is protractile

and

^retractile within the proboscis.

The

retractor muscles (D, 32) ^arise

on

the

head

^wall

and

are inserted on the stylet

and

the lever^; the protractors arise in the stipes

and

have their insertions

on

the lever.

The

structure

and mechanism

of the thrips maxilla, therefore, is practically the

same

as that of the flea (fig. 34

D),

the principal differences being in the position of the palpus

and

in the presence of a joint

between

the stylet

and

the lever in the thrips. It has been

shown by Reyne

(1927) that the maxillary stylet of the

Thysanoptera

is developed

from

a secondarily detached part of the maxilla; the musculature leaves little doubt that it represents the lacinia,

and

a reference to the cockroach (fig.

4B) shows how

readily the lacinia

(Lc)

with

its muscles _(32, 41) might

become

an independent part by separation

94 SMITHSONIAN MISCELLANEOUS COLLECTIONS

VOL. IO4

from

the

body

of the maxilla.

The

lever evidently belongs to the lacinia;in the fleas it is merely theangulated basal part of the lacinia (fig.34 G), ^intheCorrodentiaa lever isnot differentiated (fig. 10G).

The

connection of the stylet with the lever

and

the insertion of the stipital muscles

on

^{it in} the fleas

and

thrips

shows

that the lever is

not the cardo as

some

writers have supposed it to be.

The same

maxillary structure will be encountered again in the Hemiptera, but in this order the stipital lobe has

become

united with the lateral wall ofthe head,

and

the maxillary palpus ^is suppressed.

VIII.

THE SUCKING BUGS. ORDER HEMIPTERA

This order consists of the insects that entomologists regard as truebugs; its

most

unpopular

member

^is the bedbug.

The Hemiptera

are predominantly beaked insects, the proboscis of other sucking insects being seldom such a rigid structure as that of the bugs, or.

if it is beaklike, it is never in other groups a characteristic feature of theentireorder.

According

tothe structureof thewings,

though

wings are not present in all species,the

Hemiptera

are divided into

two

sub- orders,onecalled the

Homoptera

because the fore

and

hind

wings

are of similar structure, the other the Heteroptera because the

wings

are usually different.

The name Hemiptera

is used by

some

entomolo- gists for the Heteroptera alone, but by priority it includes both suborders.

Only

a

few

of the bugs are bloodsucking insects, but the blood- feeders include the widely

known

bed bug,

and some

others termed conenoses, or assassin bugs, all of

which

belong to the Heteroptera.

Most

of the species feed either on other insects or

on

the juices of plants,

and among

the latter are

many

destructive pests of cultivated vegetables,flowers,

and

fruit trees,such asthe leafhoppers, the aphids, the scale insects, the

mealy

bugs, the whiteflies, the red bugs,

and

the squash bugs. In their feeding apparatus the

Hemiptera

are the

most

specialized of all the insects, but since they have an "incomplete"

metamorphosis

they do not rank with the flies, the fleas, the beetles, the butterflies, or the

wasps and

the bees.

Though

our present interest in the order centers in the bloodsucking species, the feeding apparatusisof essentiallythe

same

structureinall.

For

a preliminary study, therefore,

we may

select one of the larger

and

better-known plant-feeding forms such as the cicada, a

member

of the

Homoptera.

GENERAL STRUCTURE OF THE HEMIPTEROUS FEEDING APPARATUS