This presentation is all about the larval forms being found in phylum arthropoda. It starts with the introduction of phylum arthropoda and then detailed information about the larval forms and their significance. i have included total 9 larval forms in this presentation.

1. Larval forms and their significance in
Arthropoda
By: Prof. Minakshi Sharma and Rekha
Dept. of Zoology
MDU Rohtak

2. INDEX
Introduction
Features of phylum arthropoda
Classification of arthropoda
Metamorphosis
Larval forms of arthropoda
Nauplius larva
Metanauplius larva
Protozoaea larva
Zoaea larva
Alium larva
Megalopa larva
Mysis larva
Schizopoda larva
Phyllosoma larva
Significance of larval forms in arthropoda

3. Phylum Arthropoda
“jointed foot”
Largest phylum
900,000 species
• 75% of all known
species
Insects, spiders,
crustaceans,
millipedes,
scorpions, ticks,
etc.

4. Features of the Phylum Arthropoda
• Paired, jointed legs
• Segmented bodies
• Bilateral symmetry
• Exoskeleton with chitin; molting
• Tubular digestive system with mouth & anus
• Open circulatory system—dorsal tubular heart
• Striated skeletal muscles
• Body cavity is a ‘blood cavity’ (blood bathes
organs)

5. Arthropoda is largest phylum in animal kingdom, which includes such familiar
forms as lobsters, crabs, spiders, mites, insects, centipedes, and millipedes.
About 84 percent of all known species of animals are members of this
phylum. Of phylum arthropoda, insecta is its largest class as shown by the
figure.

7. Life cycle of typical arthropod
Metamorphosis

8. Metamorphosis as peculiar feature of
arthropoda
 Means ‘change in form’ during development
 Arthropods do not ‘grow up’ in the same way that humans and other
vertebrates do; instead, they go through definite step-wise changes
separated by molts---adding and modifying organs and tissues and often
changing in appearance as they grow larger and mature.
 The arthropod exoskeleton is both a liability and an asset for these animals
(discuss how). Molting is the complex physiological process of periodically
replacing the old exoskeleton with a new one, permitting both increase in
size and advanced development.

9. Metamorphosis in butterfly

10. Larval forms in Arthropoda
1. Nauplius larva
• Discovered by Muller in 18th century, the Nauplius larva is the first
fundamental stage in all crustaceans that sometimes hatches from the
egg and sometimes passes inside the egg.
• Body is oval in shape and unsegmented with a large cephalothorax and
rudimentary abdomen. There are three pairs of appendages, namely,
antennules, antenna and the mandible; the last two pairs are biramous
are used for swimming. There is a single median eye.
• It has a well developed digestive system for feeding on planktons. In
Branchiopoda and Copepoda, Nauplius hatched from eggs.

11. Nauplius larva

12. Metanauplius larva
1. In some Branchiopods such as Apus, the nauplius larva
transforms into metanauplius, which is slightly larger
than nauplius and has cephalothorax and abdomen
and a caudal furca.
2. It also has a single median eye. Antennule is
uniramous and sensory but antenna is large, biramous
and locomotory in function. Mandibles reduce in size
and are used for chewing food.
3. In addition, 2 pairs of maxillae and 2 pairs of
maxillipedes make their appearance in metanauplius
for handling food.

13. Metanauplius Larva

14. Protozoea larva
1. In the case of marine prawns and lobsters, eggs hatch into
protozoea which has a large cephalothorax and elongated
unsegmented abdomen with a caudal fork and a pair of small
uropods.
2. Antennule is uniramous and segmented while antenna is
biramous. There is a single median eye.
3. Mandibles are small and masticatory in function. There are 2
pairs of maxillipedes for food gathering.
4. Three pairs of thoracic limbs make their appearance as buds.
Cephalothorax is covered by a carapace.

15. Protozoea larva

16. Zoaea larva
Zoea is the common larva of decapods and hence it has variations in its
features in different species.
It has a large cephalothorax that is covered with a helmet-like carapace that
also sports spines and it protruded into a rostrum in front.
There is one pair of compound eyes. Antennule and antenna are short and
sensory in function. First and second maxillipedes are large and biramous, used
for swimming.
Thoracic appendages are still in bud form and non-functional. Abdomen is 6-
segmented, without appendages and has a caudal furca on the tip along with a
telson.
In some Malacostraca, zoea changes in to metazoea, which grows abdominal
appendages for swimming.

17. Zaoea larva

18. Alima larva
• The alima larva of SQUILLA, which hatch out
from egg is modified zoaea.
•It is pelagic larva having glass like transparency
and long selender body with short and
broad carapace.
•Anterior cephalothorax produced into Rostrum.
•The abdomen is segmented and has four pair of
appendages and end in telson.

19. Alima larva

20. Megalopa larva
• In crabs, zoaea is tranformed into megalopa, which bears
a large cephalothroax and small abdomen bearing
small pleopods. Telson is very small.
• Antennule and antenna are small and sensory in function
and there is a pair of stalked eyes.
• The cephalothorax is covered by carapace.
• The first pair of thoracic legs is large and chelate as in
adults. The rest of the 4 pairs of thoracic legs are thin
and long and are used for crawling.

21. Megalopa larva

22. Mysis larva
• In shrimps and some lobsters zoea transforms into mysis that
resembles Mysis in general features.
•It has a cylindrical and elongated body bearing cephalothorax
and 6- segmented abdomen.
• Carapace is produced in front into a pointed rostrum.
Antennule and antenna are sensory in function.
• There are 6 pairs of biramous thoracic appendages for
locomotion and 6 pairs of abdominal appendages for
swimming, out of which the last one is modified as
uropod.
•There is a pointed telson on the tip of abdomen.

23. Mysis larva

24. Schizopoda Larva
Some crustacean decapods, such as Homarus,
Nephrops, have this larva which is similar to megalopa
and mysis. It has cylindrical body with biramous
swimming pleopods on the abdomen and biramous
appendages on cephalothorax.

25. Phyllosoma Larva
•In spiny lobsters, the egg hatches into phyllosoma larva in
which body is divisible into head, thorax and abdomen.
•There is a pair of stalked compound eyes and a pair each
of antennules and antenna as sense organs. Body is
dorsoventrally flattened and transparent.
•The first maxillipede is rudimentary and the second one is
uniramous. The third maxillipede is large, biramous and is
used for swimming.
•The abdomen is small, segmented and does not bear
appendages. Three pairs of thoracic appendages are very
long and their tips are flattened oar-like for swimming near
the surface of water.

26. Phyllosoma larva

27. Significance of larval forms
Arthropods are organisms whose body is covered with chitinous
exoskeleton for protection. But the same exoskeleton does not
allow body growth and hence must be shed in order to allow
growth. The larval stages feed and grow in order to become adults
and must undergo moulting or ecdysis to grow. After each
moulting they change their structure and size and hence are
different from the previous stage. Therefore, arthropods
demonstrates several successive larval stages before it becomes
adult.