Crustaceans, Myriapods, Insects and Chelicerates

In this article we will discuss about the external structures of Crustaceans, Myriapods, Insects and Chelicerates.

1. Head:

In Crustacea, the head is formed by the fusion of six segments. The organisation of head varies in different Crustaceans. In Cephalocarida and Branchiopoda, the head is free from thorax. In free-living Copepods, only the first thoracic segment unites with the head to form a cephalothorax. Similar condition is seen in Syncarids, Amphipods and Isopods.

In Tanaidacea, the first two thoracic segments unite with the head. All thoracic segments take part in the formation of cephalothorax in the Decapods. An exoskeletal covering, carapace, is usually present over the cephalothorax. The cara­pace is absent in Syncarids, Isopods and Amphipods.

In Cirripeds, it forms a mantle with outer calcareous plates. The carapace extends only up to the second thoracic seg­ment in Tanaidacea and Hoplocarids, up to the third thoracic segment in Cumacea and up to the fourth abdominal segment in Leptostraca and up to all cephalothoracic segments in Eucarida. In Phyllocarida, it encloses both the cephalothorax and abdo­men.

In Myriapods, a distinct head capsule is formed. In Chilopods and Symphyla, the last segment of the head bears the second max­illae which unite to form the labium. But in Pauropoda and Diplopoda this segment be­comes a part of the trunk and forms a struc­ture, called collum.

In Insects, the head is formed by the fusion of pre-antennal part and following five segments—antennal, intercalary, mandibu­lar, maxillary and labial.

The head is largely modified in different insects, but in all of them it occupies similar position and is bounded by following exoskeletal pieces— dorsal epicranium, lateral genae and frontal clypeus. Behind the epicranium there are two more pieces—occipital and post-occipital which border an opening, foramen mag­num, at the posterior end.

In Chelicerates, the head is fused with the thorax to form cephalothorax. The only exception is the Solifugids, where the two are separate. In Pseudoscorpionidae, the cephalothorax is marked by two transverse grooves.

In the Ricinulei, cephalothorax is drawn into an anterior projection, called cucullus. In all the members, an exoskeletal covering, carapace, covers the cephalothorax. The carapace is broad and strongly devel­oped in the Xiphosurida.

Structures in the Head of Insects:

Antennae:

These paired appendages work at tactile, olfactory, vibration-receptive and proprioceptive organs. They may be of various shapes—bristle-like (Locusta), moniliform (Tenebrio), club-shaped (Slipha), pecti­nate (Ctenicera) and lamellated (Melolontha).

Mouth parts:

In addition to the appen­dicular parts like mandibles and maxillae, two non-appendicular structures take part in the formation of mouth parts (Fig. 18.124).

These are called hypopharynx and labrum. The hypopharynx is the extended part of a process which originates centrally from the posterior wall of the mouth. The hypopha­rynx together with the frontal (clypeus) and ventral (labrum) exoskeletal plates in the head form a cavity in front of the mouth.

This space is called cibarium and it is inter­nally lined by a highly sensory membrane. The labrum remains movably articulated with the lower border of clypeus and acts as upper lip. The basic parts are the same in all insects but the architecture varies widely and depends upon the feeding habit of the particular group.

In Orthoptera and Coleoptera the mouth parts are like those of cockroaches adapted for biting and chew­ing. In Hymenoptera, these are modified for piercing and sucking. Here, the mandibles and the first maxillae are sharply pointed and the labium is drawn into an elongated tongue with accessory branches or paraglossae on its lateral sides.

In Hemiptera, labium encloses the elongated mandi­bles and maxillae to form a proboscis. In the females of some Diptera the mouth parts are adapted for piercing and sucking. The man­dible forms the-piercing structure and on its ventral side the labium forms the proboscis sheath to enclose six needle-shaped piercing styles.

The structures, like labrum-epipharynx and hypopharynx together with probos­cis sheath form a sucking tube. In Lepidop­tera, the mandibles are reduced in the adult. The two maxillae extend to form a long tube which remains coiled beneath the head.

2. Thorax:

The thorax may be reduced in certain crustaceans like Cladocera or un-segmented and enclosed within the carapace like Ostracoda. In free-swimming Copepods, only the first thoracic segment is indistinct and the last thoracic segment of the female is fused with the first abdominal segment.

In all the parasitic and sedentaric forms like Copepods, Rhizocephala and Cirripeds, the thorax is degenerated. In Malacostraca, the thorax has eight segments and either all or some of them take part in the formation of cephalothorax.

In Myriapods, the thorax is not differen­tiated and in all Chelicerates (exceping the Solifugids) the thorax is intimately connected with the head to form cephalothorax.

In Insects, the thorax is distinct and con­sists of three segments—prothorax, mesothorax and metathorax. In most forms these three segments are firmly united.

Structures in the Thorax of Insects:

Leg:

Each segment of the thorax bears a pair of legs. Each leg has five articles—coxa, trochanter, femur, tibia and tarsus. The distal segment may have either pad or sucking disc or single or paired claws.

The shape of the leg varies according to the habit of the insect. In walking insects, the legs are slender and in jumping insects the metathoracic insects have flattened and paddle-like legs. In the honey-bee, functional differentiation among the legs is distinct.

Wings:

The wings are not appendages; these are extensions of the body wall. In general, the insects possess one pair of wings on the metathorax. The wings develop as wing buds which may be either internal or external. Each wing is supported by an ar­rangement of branching ribs, called nervures.

Each rib carries the branches of tracheae, which remain functional up to the comple­tion of the development of wing. The shape, pattern and number of wings differ in vari­ous insects. In Lepidoptera, both the wings are large and are covered by numerous bril­liantly coloured scales. In beetles, the ante­rior part is hard and known as elytra.

At the time of rest, it covers the membranous metathoracic wings. In Diptera, the anterior pair is well-developed but the posterior pair are shortened and known as the halteres or balancers. In the Strepsiptera, the anterior wings are halteres and posterior wings are well-developed. In the bee-parasites, the an­terior pair is vestigial and the posterior pair is membranous. The wings are entirely absent in lice and fleas.

Venation:

The arrangement of veins in an insect’s wing is called venation. It is an important means for the identification of species.

The veins of a generalised wing are named as follows (Fig. 18.125):

1. Costa (C):

It is a thick, un-branched vein that forms the anterior margin of the wing.

2. Subcosta (Sc):

It is also another un-branched vein which lies below the costa (C).

3. Radius (R):

It is a branched vein lying below the subcosta (Sc).

4. Media (M):

It is a branched vein that runs medially and lies below the radius (R).

5. Cubitus (CU):

It is usually a branched vein and lies below the media (M).

6. Anal veins (A):

These are short, un-branched veins which lie below the cubi­tus (CU).

All these veins run longitudinally and are linked by cross veins which are in different lengths. The areas between the longitudinal and cross veins in the wings are called cells. The cells are numerous in may flies and dragon flies.

3. Abdomen:

In Crustaceans, the abdomen may be of various shapes. It is turned downwards and carries a pair of brood-pouches in the female Cladocera. The abdomen is four segmented in free-swimming Copepods and bears a pair of caudal styles in the last segment. The number of abdominal segments varies in Malacostraca—six segments in Decapoda, Amphipoda and Isopoda, seven segments in Leptostraca and many in Stomatopoda.

In Decapods and Isopods, the abdominal seg­ments are free but fused in Amphipoda. The abdomen in crab is much reduced and pressed along the ventral wall of the cephalothorax. In Stomatopoda, the abdo­men is longer than the cephalothorax. A telson with paired caudal styles is present in all excepting Syncarids where the caudal style is absent.

In Myriapods, the abdomen is not differ­entiated from the thorax.

In Chelicerates, the abdomen is distinct in most cases and shows the sign of further subdivision. In Xiphosura, the abdomen con­sists of six mesosomal segments and a ves­tigial metasoma. It is followed by a long and pointed telson. In Scorpionids, the mesosoma is broad and seven segmented but the metasoma is five segmented, long and nar­row. The number of segments is ten in Solifugids and Opiliones.

In Uropygi the abdominal segments are twelve. The eleven segments in the broad abdomen of Pseudoscorpionids are not separated into mesosoma and metasoma. In Ricinulei, the nine segments are fused in such a way that only four are distinctly visible. The fusion is more intimate in Araneids, where only faint lines are visible in the soft and compact abdomen. A many-jointed flagellum is seen in Palpigradida and Uropygi.

In Insects, the abdomen is made up of eleven segments. In certain forms the ab­dominal segments exhibit fusion. The poste­rior abdominal segments are often modified to form genital chambers.

Structures in the Abdomen of Insects:

The paired appendages of the abdominal segments in the insects appear in the embryo but later either fail to develop or are trans­formed into some specialised structures. Following structures are usually seen in the abdomen of many adult insects—vestigial legs, cerci and external genitalia.

1. Vestigial legs:

Larvae of pterygota bear functional appendages in the abdomen, e.g., caterpillar of Lepidoptera bears abdominal legs, called prolegs. Neuroptera larvae pos­sess gills on the abdominal appendages. But locomotor appendages are absent in the adult pterygotes. Three orders of Apterygota— Protura, Thysanura and Aptera carry some form of abdominal appendages.

In Protura, the first three abdominal segments bear ru­dimentary paired appendages resembling thoracic legs. The double appendages of Aptera in the first seven abdominal seg­ments may be either like projecting styli (Anajapyx) or like retractile vesicles (Japyx). In Thysanura, similar styli are present in the segments second to ninth and the retractile vesicles in the first seven segments of the abdomen.

Three median unpaired append­ages are present in the five segmented abdo­men of Collembola. Each appendage is formed by the fusion of the two. In the first abdominal segment, the appendage forms a structure, called collophora. In the fourth segment it becomes a spring-like furcula and in the third segment it forms a clasp, called tenaculum.

2. Cerci:

These are the appendages of eleventh and last abdominal segments. Where the number of abdominal segments is re­duced, it may also be shifted. It is small and scale-like in cockroach and grasshopper, but long and filiform in insects belonging to the Aptera, Thysanura and Ephemerida. In Dermaptera, it is modified as forceps-like structure.

3. External genitalia:

External genitalia in both the sexes are formed by the modifica­tions of abdominal appendages and other non-appendicular parts. In the female, geni­talia are concerned with the deposition of eggs and in males it assists in copulation. A number of insects possess ovipositor formed by the modification of appendages belonging to the eighth and ninth segments.

In Thysanura, paired styli and lobes work together with four elongated gonapophyses to form the ovipositor. In Orthoptera, the ovipositor is formed by two valvifers and three valvulae of eighth and ninth abdominal segments. Such ovipositor is seen in most members of Phasmida, Dictyoptera, Grylloblatodea, Corrodentia and Odonata. Numbers of valvu­lae are two pairs in Dermaptera and Neuroptera.

In Hemiptera and a few Mallophaga, it is further reduced to one pair. In Hymenoptera, all the three pairs are present but in stinging forms the entire ovipositor is modified into a sting. In Orthoptera, the ovipositor is modified for digging. In males, the abdominal appendages take part in form­ing special structures around genitalia which is used for grasping the female genitalia at the time of copulation.

It is present in Thysanura, Ephemerida, Grylloblattodea, Hemiptera, Mecoptera, Lepidoptera, Diptera, Neuroptera, Trichoptera, Hymenoptera and Siphonaptera. In all it is formed by the ap­pendages of eighth and ninth abdominal segments. In Odonata, it is formed by the modifications of the appendages belonging to the third abdominal segment.

Appendages and Non-Appendicular Struc­tures:

In Arthropods, each meta-mere contains a pair of appendages. Due to tagmatization, these appendages are often found to be shifted. Also in different forms the append­ages are variously modified according to their functions (Figs. 18.122 and 18.123). In general, the arthropod appendages are made up of several articles or segments and the body cavity extends within the appendages.

In Crustaceans, the appendages may be classified into three groups—Cephalic, Tho­racic and Abdominal appendages.

Cephalic appendages include a pair of antennules or first antennae, a pair of sec­ond antennae, a pair of mandibles and two pairs of maxillae.

(i) Antennule:

This is well developed in Ostracoda, Copepoda and Malacostraca, but small in Cladocera and extremely minute in Cirripedia. It serves as locomotory organ in Copepods where the males also use it as clasping organ. It is uniramous in Tanaidacea, Isopoda and Amphipoda. It usually bears two flagella, but in Stomatopoda the number of flagella is three.

(ii) Second antenna:

It is large in Branchiopoda, Cladocera, Ostracoda and Malacostraca. In free-swimming Copepods it is small and uniramous and totally absent in Rhizocephala.

In addition to its sensory func­tion, it may be variously modified in different crustaceans. In parasitic Copepods, it is modi­fied as a hook for adhesion and in some forms for absorbing nutrition. In Branchiopods, it works as prehensile organ and in Cladocera it is responsible for locomotion.

(iii) Mandible:

It is well formed in Cladocera, Ostracoda and Malacostraca. In the first two groups, the mandible contains a leg-like palp and a flagellum-like brush. In Malacostraca, the mandibles are with ser­rated-cutting edges which border the two sides of the mouth and a long palp to assist in ingestion.

(iv) Maxillae:

Two pairs of maxillae fol­low the mandible. The first maxilla carries a large plate in Ostracoda. The second maxilla is absent in Cladocera, and in Ostracoda it is jaw or leg-like. In Malacostraca, the exopodite of the second antenna works as foot cleaner.

Thoracic appendages vary in number among the crustaceans. The number ranges from 2-4 pairs in Ostracoda, 5 pairs in Cladocera and free-swimming Copepods to 8 pairs in Malacostraca. In Cladocera these are swimming appendages. In Ostracoda these appear as narrow legs. In free-swim­ming Copepods, the first four pairs are biramous swimming feet but the fifth pair is vestigial limbs.

Extreme variations are noted in Malacostraca. In Mysidacea, all the tho­racic appendages are alike, but in forms like Cumacea, Tanaidacea, Decapoda and Stomatopoda some of the anterior thoracic appendages are differentiated as maxillipeds and the posterior groups become walking legs.

The maxillipeds are one pair in Tanaidacea, two pairs in Cumacea, three pairs in Decapoda and five pairs in Stomatopoda. Thus the walking legs in these groups are 7, 6, 5 and 3 pairs respectively. In Syncarida, slender respiratory exopodite and double series of epipodites are in association with thoracic appendages. The Mysidacea bears only exopodites.

Abdominal appendages are absent in Cladocera, Ostracods and Copepoda. Diverse arrangements are seen in Malacostraca. In Leptostraca, the first four pairs are biramous swimming feet and the last two pairs are uniramous and insignificant. The last pair in Syncarida together with telson constitutes a fan-shaped uropod. The first five pairs in males Mysidacea are larger than those of females.

In Amphipoda, the first three are biramous swimming appendages but the last three are modified for jumping. First few in Isopoda and first five abdominal append­ages in Stomatopoda carry the gills and in both the last pair form the uropods. Well- developed uropods are seen in Decapods like prawn and lobsters, but is absent in true crabs.

In Myriapods, the appendages are classi­fied into two groups—cephalic and trunk ap­pendages.

Following appendages are present in the Cephalic region—antennae, mandibles and maxillae:

(i) Antennae:

In most cases, one pair of elongated and many jointed antennae are present. In Pauropods, the antennae are branched.

(ii) Mandibles:

These paired structures are used for cutting the food. Here, the mandibles are without palp.

(iii) Maxillae:

In Chilopods and Symphyla, the maxillae are two pairs and the second maxillae unite to form the labium. In Pauropoda and Diplopoda, the second max­illae are absent.

Trunk appendages are represented by the legs. In Chilopoda and Symphyla, each trunk segment is provided with a pair of jointed legs. The legs of the first segment are directed anteriorly and through its terminal end open the duct of the poison gland.

It is known as poison jaws or maxillipeds. In Diplopods, each segment after the first four possesses two pairs of legs. The legs of the seventh segment are modified in male as copulatory organs.

In Chelicerates, the appendages are cephalothoracic and abdominal.

Three types of appendages are found in the Cephalo­thorax:

(i) Chelicerae,

(ii) Pedipalpi and

(iii) Walking legs.

(i) Chelicerae:

It is the appendage of first post-oral segment, but it occupies pre-oral position. In Scorpionids, it is small and com­posed of three articles. The distal articles are chelated.

It is two-jointed and hook-like in Amblypygi. Number of articles varies in Acarida from 2-6 and this appendage may be pincer-like, fang-like or lance-like. The chelicerae contain silk glands in Pseudoscorpionids and poison glands open through them in Araneida. It is quite promi­nent in Palpigradi, Ricinulei and Solifugae.

(ii) Pedipalpi:

It represents the second post-oral segment. In Xiphosurida, it is leg-­like, jointed and helps both in food capture and locomotion. Here the proximal segment is spiny and the distal part is chelate. In Scorpionida, Araneida and Pseudo­scorpionids, the pedipalp is made up of six articles. It is leg-like in Palpigradi, Solifugae and Opiliones.

In Uropygi, it is small, stout and its base, together with the upper lip of the rostrum, forms a ball and socket-joint around the mouth to act as filtering appara­tus, In Amblypygi, each pedipalp is seven segmented and is raptorial in function. It contains poison gland in Pseudoscorpionids, and in Acarida it may be variously modified. In male Araneida, the tip of the pedipalp is modified for sperm transfer.

(iii) Walking legs:

In the Xiphosurida, the third to sixth pairs of appendages are the walking legs. The legs exhibit sexual dimor­phism. In addition to their function of loco­motion the legs are adapted for ingestion. For this purpose, the bases of the legs are drawn into spiny lobes, called gnathobases.

All the legs excepting the last are chelated. The last leg bears four movable spines at the distal tip to act as shovel and a spatulate process in the outer border of its base.

In addition to the walking legs, the cephalothorax of Xiphosurids bears a pair of small flat appendages, called chillaria. All Archnids contain four pairs of legs. In Acarida, the legs are clawed at the distal tips. In Opiliones the legs are very long and slender. In Uropygi and Palpigradi, the sec­ond pair of legs is long and antenna-like.

Abdominal appendages in the Chelicerates usually do not take part in loco­motion. The Xiphosurida contains six pairs of abdominal appendages on the ventral side of the abdomen. Each lamellar appendage carries a slender inner process and a broad outer plate. First pair is fused in the middle to form a genital operculum.

The remaining pairs are free and each carries a gill. In Arachnids, the abdominal appendages ap­pear as rudiments during embryonic devel­opment. But in adults these are transformed into various structures like pectines in Scorpionids and spinnerets in Araneida.

The respiratory organs, book-lungs, are also formed as the appendage buds near the posterior end of the abdomen of the embryo. The caudal spine of Xiphosurids is regarded as the appendage of the telson, and the poison gland and sting of Scorpionids are formed by the transformation of the telson.

In Insects, various structures are grouped as cephalic, thoracic and abdominal ones.

The cephalic region includes:

(i) Antennae and

(ii) Mouth parts.

The thoracic region contains locomotor structures:

(i) Legs and

(ii) Wings.

The abdomen in adults do not have any paired appendage, but various structures like stings, ovipositors, genital processes, etc. are seen to be present in the abdo­men. The eye is often considered as an ap­pendage of pre-antennal segment. The wings and other un-jointed structures having no supply of blood, nerve and muscles are re­garded as non-appendicular structures.