• Some structural and biological patterns which have
contributed to the big biological success of arthropods
are :- Thick chitinous exoskeleton
- jointed body and limbs
- Locomotion by small, striated, extrinsic muscles
- Various specialisations of somites, alimentary canal
and respiratory organs - Cephalisation with concentration of ganglia and
sense organs in head region - Behaviour patterns with primitive intelligence and
social instincts in some group.
reasons for biological success of arthropodaScientific study of crustaceans is known as carcinology
and acarology is the scientific study of ticks and mites.
Entomology is the scientific study of insects.• Characteristics of some important classes of Phylum
Arthropoda are summarised in a table on next page.
Class inseCta
• Class Insecta of Phylum Arthropoda is the largest in the
animal kingdom, as there are more species of insects (about
7,50,000) than all other species of animals combined.
• Insects are air breathing and usually flying arthropods.
They are distinguished from other arthropods by the
possession of 3 pairs of legs and usually 2 pairs of
wings which may be absent in some.
• Body made up of head (6 fused segments), thorax (3
segments) and abdomen (upto 11 segments).• Head with compound eyes (1 pair), antennae (1 pair),
mandibles (1 pair) and maxillae (2 pairs).
• Class Insecta, Diplopoda and Chilopoda can be grouped in a
taxon called tracheata due to presence of tracheal system
as respiratory organ.
types of mouth parts in insects
• Mouth parts in insects are modified for different feeding
habits.
• In general, mouth parts of an insect
consist of one labrum, one
labium, one hypopharynx,
two mandibles and two
maxillae. As insects feed
on animals and plants in a
diversity of ways, their mouth
parts are modified for these
purposes. Important types of
insect mouth parts are described as
follows :
metamorphosis in insects
• In case of insects where development is usually indirect the
egg hatches into a dissimilar larva instead of a miniature
adult.
• Transformation of an immature larval individual into a
sexually mature reproducing adult of very different form,
structure and habit, is called metamorphosis.
• Insects display four types of metamorphosis:- No-metamorphosis or ametabolous development
: In ametabolous development, metamorphosis does
not occur. These insects are most primitive and
wingless. E.g., silver fish, spring tails.- Incomplete metamorphosis or
hemimetabolous development
: In this type of metamorphosis, the
life history includes egg, naiad
(young) and imago (adult).
Young is aquatic and respires by
tracheal gills, whereas, the adult is
aerial and respires by tracheae. E.g.,
mayflies, dragon flies, and stone flies.
- Incomplete metamorphosis or