2. • Thorax - Second tagma - locomotion
• Thorax - pro, meso and meta thorax
• All the thoracic segments bear legs
• Pterothorax bears wings
• Larval Apocrita, Diptera, Coleoptera – apodous
• Larval stage – flexible; adults – hardened cuticular
plates
THORAX
3. • Holometabolous larvae - cuticle is flexible
• Longitiudinal muscles are attached to the
intersegmental folds
• Move as a result of successive changes in the
shapes of the thoracic and abdominal
segments permitted by the flexible cuticle
• Cuticle is sclerotized in adult - longitudinal
muscles are fused with the segmental sclerites
behind
4. • Thorax - composed of a number of sclerites (cuticular
plates)
• The dorsal sclerites are collectively called the notum or
tergum
• Lateral sclerites are called the pleuron
• Ventral sclerites are called the sternum
5. Tergum
• Prothoracic tergum – pronotum
• In most insects, it is small and serves for muscle
attachment of legs
• In Orthoptera, Dictyoptera & Coleoptera - large
saddle / shield shaped & protects pterothorax
• Pronotum mimics environmental characters. eg.
horn in Treehopper, cow bug
• Pronotum is modified into elongate projection over
the head – eg. Hercules beetle
• The narrow intersegmental sclerites of thorax join
with the segmental sclerite behind
6. • Strengthening ridges develop internally in tergum
• Externally they appear as sulci
• The groove of the intersegmental sclerite (ridge /
groove) - anticostal sulcus - well developed –wing
usage – phragmata – muscle attachment
• The narrow rim in front of the sulcus – acrotergite
• Acrotergite never occur at the pronotum, as it
forms the neck
• Each acrotergite and anticostal sulcus together -
post notum
7. • When both wings used for flight - mesopostnotum and
metapostnotum present
• Hind wing only used - only the metapostnotum is
developed (Orthoptera and Coleoptera)
• Mesopostnotum only in Diptera (forewing is used)
• The antecostal ridges at the front and back of the
mesothorax and the back of the metathorax develop
into extensive internal plates called phragmata
• Depending upon the usage of the wing the respective
phragmata are developed
8. • A transverse sulcus (prescutellar sulcus) divides
notum into an anterior prescutum and scutum
(Aliscutum)
• A ‘v’ shaped sulcus (scuto-scutellar) posteriorly
separates the scutellum from scutum
10. Summary of Thoracic nota
Dorsal body plate of each thoracic segments are called
as pronotum, mesonotum and metanotum respectively.
Pronotum:
This sclerite is undivided and saddle shaped in
grasshopper and shield like in cockroach.
Pterothoracic notum:
Have 3 transverse sutures (antecostal, prescutal and
scuto-scutellar) and 5 tergites (acrotergite, prescutum,
scutum, scutellum and post-scutellum).
11. Sternum
• Ventral sclerites are segmental and intersegmental -
often remains separate in the thorax
• The intersegmental sclerite is produced internally
into a spine - the spinasternum
• The segmental sclerite – eusternum
12. • Eusternum is divided into basisternum and
sternellum by a sulcus
• A pair of sternal apophyses arises from the
eusternum externally marked by pits
• In higher insects both the sternal apophyses arise
together in the midline - internally form a ‘y’ shaped
furca
14. • The sternum of the pterothoracic segment does not
differ from prothorax
• But the basisternum is bigger in flying insects,
provide attachment for dorsoventral flight muscles
• Gives rigidity to thorax
15. Summary of Thoracic sterna
Vental body plate of each thoracic segments are called
as prosternum, mesosternum and metasternum.
Thoracic sterna is made up of a segmental plate called
eusternum and an intersternite called spinasternum.
Eusternum is made up of three sternites viz.,
presternum, basisternum and sternellum.
16. Pleuron
• The pleural regions are membraneous in many larval
insects, but sclerotized in adult
• Made of 3 slcerites
• One ventral sclerite or sternopleurite – articulates with
coxa
• Two dorsal sclerites – anapleurite and coxapleurite –
both are separate in Apterygota and in lower
pterygotes
• In other groups both anapleurite and coxapleurite are
fused to form pleuron
17. • Pleuron develops a vertical strengthening ridge,
above coxa
• Pleuron ridge externally marked - pleuron sulcus
• It divides pleuron - anterior episternum and
posterior epimeron
19. Summary of Thoracic pleura
Lateral body wall of thoracic segment between notum
and sternum.
Selerites of pleuron is called as pleurite and they fuse
to form pleural plate.
Pleural plate is divided into anterior episternum and
posterior epimeron by pleural suture.
Pterothoracic pleuron provides space for articulation
of wings and legs
20. Functions of thorax
Mainly concerned with locomotion
Thoracic appendages
Three pairs of legs and two pairs of wings
Two pairs of spiracles in the mesopleuron and
metapleuron
21. • Insects have three pairs of legs, One pair each in
pro-, meso- and metathorax - ‘Hexapods’
• Each leg consists of six segments, articulating with
each other by mono-or-dicondylic articulations set
in a membrane, the corium.
LEGS
23. • Coxa – Basal segment – like a truncated cone and
articulates basally with the wall of the thorax.
• Articulation - at single point with pleuron -
movement of coxa is very free.
• In some, the 2nd articulation with the trochantin,
restricts the free movement to some extent.
• In some higher forms - rigid pleural and sternal
articulations limit the movement of coxa
• In Lepidoptera the coxae of middle and hind legs,
and in coleoptera - Adephaga hind coxae are fused
with the thorax.
24. • The trohanter is small with dicondylic articulation
with coxa - can move in the vertical plane
• In odonata and Ichneumonids - two segmented
trochanter
• The 2nd one is part of the femur
• The femur is small in larval insects
• In most adults - largest and stoutest part of the leg
• It is fixed to the trochanter and move with it.
• There are no muscles to move femur with respect to
trochanter.
25. • The tibia is the long part of the leg articulating with
femur by a dicondylic joint - vertical plane movt.
• In most insects - the tarsus - two to five tarsomeres
• Not true segments - lack muscles.
• The basal tarsomere (basitarus) - articulates with
the distal end of the tibia - single condyle.
• Between tarsomeres - no articulation
• Connected by flexible membrane and are freely
movable.
26. • In Protura, Collembola and larva of most
holometabolous insects - tarsus unsegmented
• Pretarsus - median membranous / partly
sclerotized lobe called arolium
• A pair of claws - articulates with a median
process of the last tarsomere – Unguifer &
• Ventrally – a sclerotized plate called
Unguitractor
• Small plates Between Unguitractor and the
claws - auxiliae
27. • In Diptera except Tipulidae - no
arolium
• a membranous pulvillus arises from
the base of each auxilia &
• A spine / lobe like empodium arises
from the Unguitractor
• Claw development varies
• In thrips, claws are minute & in
pretarsus - the bladder like arolium
present
• In Mallophaga – differential claw
devt. – one claw can be seen
29. • Leg pads - walking on smooth surface
• Claws give needed grip - walking on rough surface
• When one structure is used, the other is bent
upwards.
30. Silk production: e.g. Embid foreleg
• In embiids, the basal tarsomere is greatly swollen
• In this silk glands (upto 200 glands) connected by
duct with a pore at the tip - silk is exuded.
31. Reduction of legs (Adult stage)
(i) Nymphalid butterflies:
In males the tarsus and pretarsus of foreleg are
lacking
In females the fore tarsus consists of very short
tarsomeres
(ii) Female coccids:
Sedentary - held in position by the stylets. The
legs are reduced.
(iii) Female stylopids:
Legs absent and are parasitic
32. Reduction in leg (Larval stage)
(i) Leaf miners of Lepidoptera, Coleoptera and
Sawflies
(ii) Parasitic larvae of Hymenoptera and Strepsiptera
(iii) Grubs of blister beetles
(iv) In Hymenoptera - larvae fed by the adults are
apodous
33. Types or modifications of legs
Based on the habitat and food habit
1. Walking/ Ambulatorial leg
2. Running / Cursorial leg
3. Jumping / Saltatorial leg
4. Grasping / Raptorial leg
5. Digging / Burrowing / Fossorial leg
6. Swimming / Natatorial leg
7. Clinging / Scansorial leg
8. Honey bee’s legs/ Foragial leg
35. Wing development in modern insects
• Adult insects - fully developed wings
• In holometabolus larvae - develop internally
• In hemimetabolous larvae - visible as external pads
• Mayflies have two winged stages viz., subimago and
imago
36. • Each wing is a hollow extension of the body wall
• It arises dorsolaterally between the pleura and
nota of meso and metathoracic segments.
• The pleural plates of the thoracic segments -
support the bases of the paranotal lobes
37. • The wing membrane is formed by closely apposed
two layers of integuments
• Two layers remain separate at certain places and the
cuticle - thicker and heavily sclerotized – form vein
• Each vein have a nerve, a tracheae & have 2 rows of
trichoid sensilla
• Haemolymph can flow into the wings as the cavities
of the veins are connected with the haemocoel.
39. VEINS AND VENATION
• A number of longitudinal and cross veins support the
wing membrane
Comstock-Needham system of venation
1. Costa (C) - the leading edge of the wing
Subcosta (Sc) - second longitudinal vein (behind the
costa) – unbranched
2. Radius (R) - third longitudinal vein, one to five branches
reach the wing margin
3. Media (M) - fourth longitudinal vein, one to four
branches reach the wing margin (MA, MP)
4. Cubitus (Cu) - fifth longitudinal vein, one to three
branches reach the wing margin (Cu A, CuP)
5. Anal veins (A1, A2, A3) -- unbranched veins behind the
cubitus
40. c-sc crossveins run between the costa and subcosta
r crossveins run bewteen adjacent branches of the
radius
r-m crossveins run between the radius and media
m-cu crossveins run between the media and cubitus
41. Modification of venation
• Any of these veins may branch and given with
subscripts 1, 2, 3, etc.
• These branches are not homologous in different
groups of insects.
• In some small insects the venation may be reduced
• In Chalcid wasps - the subcosta and part of the radius
are present
42. • Increase in venation by branching of existing veins
• Development of additional intercalary veins occur in
Orthoptera
• Large number of cross veins may be present to form
the reticulum - in Odonata, bases of forewings of
Tettigonidae and Acrididae.
• In fossil insects irregular networking of veins -
archidictyon
43. Areas of the wing
• The flexion and fold-lines divide the wing into
different areas.
• The region containing the bulk of the veins in front of
the claval furrow is called remigium.
• The area behind the claval furrow is called clavus
• In hind wings in which this area is greatly expanded
and known as vannus & the fold – vannal fold / plica
vannalis.
• The jugum is cut off by the jugal fold / plica jugalis if
it is present
45. Margins and angles
• triangular in shape – 3 sides & 3 angles
• anterior margin strengthened by the costa - coastal
margin
• lateral margin - apical margin
• posterior margin - anal margin
• angle of attachment to the thorax - humeral angle.
• angle between the coastal and apical margins - apical
angle
• angle between apical and anal margins - anal angle.
Humeral
angle
Costal
Margin
Apical
angle
Apical
margin
Anal angle
Anal
margin
Claval
furrow
Juga
fold
46. Pterostigma
• Pigmented spot on the anterior margin of the wing
• Present on both pairs of wings in Odonata
• In the forewings of many Hymenoptera, Psocoptera,
Mecoptera and Neuroptera.
• In Odonata, - it reduce the wing flutter during
gliding and thereby increases the gliding speed.
47. Wing membrane
• The wing membrane is semitransparent and exhibits
iridescence
• Sometimes wings are patterned by pigments in the
epidermal cells eg. mecoptera and tephritidae
• In insects with hardened forewings such as
Orthoptera and Coleoptera, the whole wing is
pigmented
48. • In Trichoptera larger macrotrichia covers the
whole of the wing membrane giving it a hairy
appearance
• In Lepidoptera the wings are clothed in scales
• Scales also occur on the wing veins and body of
mosquito & on the wings of some Psocoptera,
Trichoptera and Coleoptera.
49. higher pterygote insects, fore & hind wings -
coupled together as a unit
both pairs move synchronously
Types of wing coupling
1. Hamulate :
• A row of small hooks - on the
coastal margin of the hind
wing - hamuli.
• engage the folded posterior
edge of fore wing.
• e.g. bees.
WING COUPLING
50. 2. Amplexiform :
• the simplest form of wing coupling
• A linking structure is absent.
• Coupling is achieved by broad overlapping of adjacent
margins.
• e.g. butterflies.
51. 3. Frenate :
There are two sub types. e.g. Fruit sucking moth.
i. Male frenate :
• Hindwing bears near the base of the coastal margin
a stout bristle called frenulum
• held by a curved process, retinaculum arising from
the subcostal vein found on the surface of the
forewing
52. ii. Female frenate :
Hindwing bears near the base of the costal margin
a group of stout bristle (frenulum)
engages there in a retinaculum formed by a patch
of hairs near cubitus.
53. 4. Jugate :
Jugam of the forewings are lobe like and it is locked
to the coastal margin of the hind wings.
e.g. Hepialid moths.
