Entomology 2002

Review for

External Anatomy of the Insect Head

Review Topics• Lateral view of the generalized head

• Frontal view of the generalized head

• Sclerotized structures inside the head

• Visual organs

• Antenna structure

• Mouthparts

• Ventral view of the mouthparts

Head Anatomy – Lateral ViewSclerotized regions Sclerite names Sutures Condyles

prot

hora

x

frontal

suture

subgenal suture

post

occi

pita

l sut

ure

labrum

clypeus

mandible (2) maxilla

(2)

labium

gena

vertex

occi

put

frons

subgena

post

occi

put

antennal socket (2)

ocellus (3)

compound eye (2)

posterior tentorial pit (2)

anterior tentorial pit (2)

hypopharynx

antennifer(antennal condyle)

mandibular condyle (2)

Review Topics

Generalized Head: Frontal View

Review Topics

gena

vertex

frons

subgena

labrum

clypeusmandible

frontal suture

subgenal suturefronto-clypeal suture (epistomal suture)

antennal socket (2)

compound eye (2)

ocellus (3)Don’t click yet!!! The

thing is alive!!Click after the ‘feelers’ are

gone and it doesn’t have any more feelings!

Insect Head - Oblique Viewpartial dissection to show tentorium

Not shown for clarity:Internal tissues: Muscle,

nervous, digestive

External: conjunctivae

Tentorial structures are sclerotized– Internal strength

to the head– Additional

attachment for various muscles

dorsal tentorial arm

clypeuslabrum

fronsm

andi

ble

compound eye

po

sto

ccip

ut

genaantennaeposterior tentorial pit

anterior tentorial pit

tentorial bridge

anterior tentorial arm

occipital foramen

Review Topics

Insect Photoreceptor Organs• Compound eyes

• Paired, symmetric• Composed of grouped

ommatidia • Between 1-30,000/eye

• Image-forming• Where visual fields

overlap the insect has depth-perception

• Variable capability to perceive different wavelengths• Apis cannot see red, but

can see UV

• Ocelli• Usually 3 (otherwise none

or two present)• On the frons or vertex of

many adults and nymphs• Single lens; not image-

forming• Stemmata

• In two groups on side of head capsule

• Found only in immature holometabolous insects

• Structurally intermediate between compound eyes and ocelli

• Image-forming when grouped

Review Topics

Structure of the Ommatidium• Light Light striking the corneal

lens and crystalline cone is deflected to the rhabdom

• The light signal is converted into nervous impulses by retinula cells

rhabdom

secondary pigment cell (12-18)

corneagen cell (2) (primary pigment cell)

crystalline cone

corneal lensLight-gathering apparatus

Light-sensing apparatusretinula cell (~8)

•Pigment can be concentrated within the pigment cells to allow more light entry under low light conditions

Review Topics

Many Ommatidia are Grouped to Form the Compound Eye

Close-packed arrangement of the ommatidiaEvidenced by the

hexagonal appearance of external surface of eye

Each ommatidium is angled slightly from neighbors to permit wide-angle views

Nervous inputs from all the ommatidia are integrated within the optic lobe and the protocerebrum

Review TopicsCompound eye of Pterophylla camellifolia

Ocellus structure• Single cuticular lens• No crystalline cone• Multiple rhabdoms and

sense cells• Ocellar nerve innervates

deutocerebrum

Other PhotoreceptorsStemma structure• Single lens• Crystalline lens usually

present• Multiple rhabdoms• Stemmata arranged in

image-forming clusterscorneal lens

ocellar nerve

rhabdom

retinula cell nucleus

corneagen cell nucleuscuticle

pigment

Review Topics

corneagen cell nucleus

corneal lens

proximal rhabdom

distal rhabdom

crystalline lens

sense cells

Structure of the AntennaGenerally 3 parts:

Scape: basal most segment

Pedicel: contains chordotonal organSense vibrations (sound in ♂ mosquitoes)

Flagellum: 1-30+ segments

Sensilla of several types presentA given sensilla type usually concentrated in a

particular region of the antenna

Sensilla counts range from <8K to >250Kflagellum

pedicel

scape

Review Topics

Head capsule

Movement of the Antenna

flagellum

pedicel

scape

Review Topics

antennifer(single condyle: ball-and-socket type movement)

Head capsule

Only the scape has any internal musculature Exception in Diplura, which has musculature

throughout

Contraction of muscles originating on head capsule inner wall and inserting on scape cause scape & more distal segments to move

Muscles originating in scape and inserting on pedicel move pedicel and flagellum

Monocondylic attachment means antenna rotates freely about point of articulation

Generalized Insect MouthpartsMandibles

• Paired, highly modified and sclerotized appendages capable of grinding food before ingestion

LabrumUnpaired, articulates with

clypeus by narrow membraneAllows considerable

movementForms “upper lip”

grinding region

incising region

Axis of

rotation

Mandibular adductors attach to this apodeme

Mandibular abductors attach to this apodeme

Two mandibular condyles confine the

movement of the mandible to a single

axis of rotation

mesad

distad

Review Topics

clypeus

labrum

midline

Generalized Insect Mouthparts (cont.)Labium

Unpaired, moderately modifiedOne can envision two maxillae

fused along the midlineActs as “lower lip”

submentum

mentum

paraglossum

glossae

prementum palp

ligula

midline

Maxillae• Paired appendages with

several distinct sclerites, capable of further manipulating food

mesad

distad

cardo

stipes

laciniagalea

palp

Single condyle: movement not

confined to single axis

Two labial condyles confine the movement of the labium along a single axis of rotation

Generalized MouthpartsVentral View

Labrum

Anterior

Posterior

Mandible

Maxilla

Labium

Hypopharynx

Mouth

Salivary gland

opening

Cibarium

SalivariumReview Topics

Not GeneralizedPterophylla camellifolia

(true katydid)

Review Topics

Frons

Gena

Maxillary palp

Mandible

Labrum

Galea of Maxilla

Labial palp