Download II. How did insects evolve and further differentiate into current groups?

Entomology: April 5, 2006
I.
Evolutionary relationships of the insects
A.What are arthropods?
1.
Basic arthropod characteristics
a)
body segmentation, generally with specialized regions (tagmata: i.e. head,
thorax and abodomen)
b)
chitinous exoskeleton
c)
paired, jointed appendages
2.
Arthropod groups
a)
Subphylum Unirama: insects, centipedes, millipedes
b)
Subphylum Crustacea: decapods, isopods, cirripeds (barnacles)…
c)
Subphylum Chelicerata: spiders, mites, scorpions, horseshoe crabs...
d)
Subphylum Trilobita (extinct...)
B. Where do Arthropods fit in the overall phylogeny of Kingdom Animalia?
1.
Most related to annelids and molluscs
C.What are the arthropod ancestors?
1.
Arthropods arose either from an annelid, or from a shared ancestor with the
annelids
a)
Key features shared with annelids

Segmentation

Paired (fused) ventral nerve cords with ganglia associated with
segments (annelid has one per segment, arthropods generally have
more consolidation of these ganglia)

Both have a cuticle

Both are protostomes (as are the molluscs)

We will discuss these features later in the course
Features which differ from Annelids

Only arthropod cuticle is hardened with chitin and scleratinized

Arthropods show clear tagmatization

Arthropods have paired and jointed appendages

Arthropods have open circulation, while annelids have closed
circulation. However, other aspects of circulatory system are similar

Arthropods have a reduced coelom

PLUS many other differences as arthropods became more
differentiated
c)
No good Precambrian fossils which could be useful in elucidating ancestry,
so proposed relationship is "theoretical"

Rely on comparing external features of more recent fossils, as well
as molecular tools.
d)
Onycophorans as possible intermediate forms?
Are arthropods monophyletic (derived from a single common ancestor)?
a)
Pro: Many shared characteristics between arthropod groups.

basic features above

More specific aspects of circulatory and respiratory systems

compound eyes

sperm ultrastructure

rRNA sequences and other molecular evidence
b)
2.
b)
Con: Some basic differences in appendage structure

Biramous (two-branched) vs. uniramous (single branched)
3.
Proposed evolution of insects from annelid ancestor (Fig. 11.12)
a)
Basic annelid or annelid-like ancestor

Note prostomium and periproct not considered true segments
b)
Next step, similar to Onycophorans (rare but living, something like a
“missing link”)

Development of paired, bilateral (non-segmented) appendages for
locomotion

Simple (non-segmented) antennae

Simple eye (also common in Polychaetes)
c)
Arthropodization and other changes:

"Arthropodization": appendages, including antennae, become
segmented

Forward segments (1-4) become compressed and associated with the
prostomium. (Disputed, might be up to the first 7 segments.)

Compound eye develops (possibly from clusters of simple eyes)
d)
Further development of features in c:

Forward segments even more compressed

Appendages on the forward segments become more reduced and
associated with the head

Further development of compound eye
e)
Continued development…

Continued compression of segments to form head region

Appendages of first several segments have become typical
mouthparts

NOTE: At this stage, similar to centipedes and millipedes.
f)
Trend toward increased specialization of body regions: head, thorax and
abdomen

locomotory appendages restricted to the three thoracic segments

most abdominal appendages reduced/gone

modification of ab segs 8-9 into genitalia

cerci remain
II. How did insects evolve and further differentiate into current groups?
A.Entognathous apterygotes were the earliest insects
1.
Entognathous: mouthparts pulled into head
2.
Apterygotes: “without wings”
B. Development of ectognathous forms
1.
Ectognathous: mouthparts extended
a)
Derived condition (i.e. thought to have appeared later in evolution)
b)
All common insects have this.
2.
Ectognathous apterygotes include
a)
Thysaneura: silverfish, bristletails
b)
Archaeognatha: jumping bristletails
C.Development of winged forms, the Pterygota [see Fig. 11.4 to follow evolution from this
point onward]
1.
First Pterogote fossils in carboniferous,
2.
No known intermediates
3.
Developed without loss of legs (unlike birds and bats…)
4.
From what structure are they derived?
a)
Hyopoth. 1: Expansion of notum

control fall from vegetation

courtship display?

further development of joints, musculature
b)
Hyopoth. 2: Gill origin

already moveable

explains lack of biramous appendages as found in other arthropods
5.
First wing types were held laterally, unable to fold over abdomen at rest
(=Paleopterous condition or “primitive winged”)
a)
Paleopterous ("primitive-winged" ) orders:

Odonata (dragonflies and damselflies)

Ephemeroptera (mayflies)
D.Evolution of wing flexion (neopterous insects)
E. Development of complete metamorphosis (endopterygotes)
1.
Those with “simple” or incomplete metamorphosis are the exopterygotes
III. Begin Pterygote (winged insect) orders
A.Paleopterous exopterygotes (primitive-winged, incomplete metamorphosis)
1.
Order Odonata: dragonflies and damselflies
a)
Suborders

Suborder Anisoptera: dragonflies

Suborder Zygoptera: damselflies

Key differences

Dragonflies rest with wings out, Damselflies rest with wings
extended outward above abdomen (remember, they can’t fold
them flat over abdomen)




Dragonflies are stronger fliers (up to 35 mph!)
Dragonflies have more robust bodies
Dragonflies have larger, broader hindwings than forewings,
whereas the two pairs are nearly equal in the damselflies
Dragonfly larvae move via “jet propulsion”, forcibly ejecting
water through their rectum

b)
Compound eyes close together dorsally in dragonflies (both
have excellent vision!)
Other characteristics

Despite the label of “primitive-winged”, they are some of the best
fliers in the insect world: fast, able to hover, fly backwards

Each pair of wings used separately (in most orders, forewings and
hindwings are hinged together and work as one unit)

Aquatic, predatory larvae; adults usually found near water




Highly extendable labium (lower jaw) helps them catch prey
Adults prey on flying insects, capture by extending forelegs
Adults mate in flight
Earliest dragonflies were gigantic, with wingspans up to 30 inches!
Study questions
1.
What are the three basic characteristics of Phylum Arthropoda?
2.
Name the three Subphyla of living arthropods, and representative members of each.
3.
Describe several features that members of Phylum Annelida and Phylum Arthropoda have
in common. Then, describe several features that differ between the two phyla.
4.
What is meant by “monophyletic”? What evidence suggests that the arthropods are a
monophyletic group?
5.
Describe the proposed evolutionary changes that transformed an annelid-like ancestor into
an insect.
6.
Once the basic insect body plan was developed, there were four major developments that
occurred during the course of insect evolution. List and briefly describe each of those
developments.
7.
Name two orders of insects that are Paleopterous exopterygotes. (And of course, know
what is meant by “paleopterous” and “exopterygote”.)
8.
What are the two suborders of the Order Odonata? Describe several differences between
these suborders.
9.
Does being “primitive winged” mean that the Odonata are inferior fliers? Explain. Also,
how does the way Odonata use their wings in flight differ from the method of flight in
most other insects?
10.
What is the key food source of the Odonata in the adult stage? In larval stages? How do
the adults catch their prey? How about the larvae?
11.
Describe a unique mode of movement for dragonfly larvae.