11. Study of Animal Type - Cockroach - part 01 - External morphology

 

11. Study of Animal Type - Cockroach - part 01 - External morphology

Habit and habitat :

• Cockroaches are omnipresent i.e. present everywhere, all over the world. 
• It prefers damp and moist places, crevices to live.
• It is omnivorous, nocturnal and cursorial. 
• Three common species of cockroach found in India are-

1. Periplaneta americana
2. Blatta orientalis and 
3. Blatta germanica  

Systematic Position :

• Kingdom : Animalia (Cell wall absent, heterotrophic nutrition.)
• Phylym : Arthropoda (Jointed appendages are present, segmented body, chitinous exoskeleton.
• Class : Insecta (Two pairs of wings and three pairs of walking legs are present.)
• Genus : Periplaneta (Nocturnal, straight wings.)
• Species : americana (Origin is in Continent of America)

External morphology :
Shape and size : 

• Cockroach has an elongated, bilaterally symmetrical and dorso-ventrally flattened body. 
• They are triploblastic, eucoelomate and truly segmented animals.
• Body cavity is called as haemocoel, filled with haemolymph.

Coloration : 

• Cockroach is glistening brown or red brown in colour.

Exoskeleton : 

• Body of cockroach is protected by hard, waxy, tough, non-living exoskeleton.
• Exoskeleton is formed by a nitrogenous polysaccharide-chitin that provides strength, elasticity and surface area for attachment of muscles. 
• Each body segment of cockroach is covered by four chitinous plates (sclerites) namely, 

1. dorsal tergum
2. ventral sternum and 
3. two lateral pleurons.

Body Division : 

• Body is divided into three regions namely

1.  head
2. thorax and
3. abdomen.

• Head is attached at right angles to thorax by a thin, narrow, movable neck or cervix.

01. Head : 

• It is formed by the fusion of six segments. 
• It is triangular or ovate in shape. 
• It is highly mobile due to flexible neck. 
  
• It bears a pair of long antennae, a pair of compound eyes and mouth parts adapted for chewing and biting of food.

1. Antennae : 

• Also called as feelers. 
• There are filamentous, long, segmented structures that can be moved in all directions. 
• They are lodged in membranous pits called antennal sockets.
• They are tactile (touch) as well as olfactory (smell) organs, useful in locating the food material in the vicinity.

2. Fenestrae

•  are also called as ocellar spots situated at the base of each antenna and they appear as white spots.

3. Compound eyes : 

• They are paired, dark, kidney shaped structures placed on lateral sides of the head and are made up of large number of ommatidia (singular ommatidium).
• Ommatidia are structural and functional units of compound eye, each forming an image of very small part of visual field. 
• Collectively,compound eye produces a mosaic image.

4. Mouth parts : 

• Cockroach has pre-oral cavity in front of mouth in which foods is received.
• It is bounded by chewing and biting type of mouth parts. 
• These are movable, segmented appendages which assist in ingestion of food.

Mouth parts of cockroach comprise of....

i. Labrum (Upper lip) : 

• It is single flap-like movable part which covers the mouth from upper side. 
• It forms an anterior wall of pre-oral cavity. 
• It is useful in holding of the food during feeding.

ii. Mandibles (True jaws) : 

• These are two dark, hard, chitinous structures with serrated median margins. 
• They are present on either side, behind the labrum. 
• They perform coordinated sidewise movements to cut and crush the food. 
• This movement is effected with the help of adductor and abductor muscles

iii. Maxillae (Accessory jaws) : 

• These are also called as first pair of maxillae. 
• These are situated on the either side of mouth behind the mandibles. 
• Each maxilla consist of sclerites like 

1. cardo
2. stipes
3. galea
4. lacinia and 
5. maxillary palps. 

• Maxillary palps act as tactile organs. 
• The maxillae hold food, help the mandibles for mastication. 
• Maxillae are also used for cleaning the antennae and front legs.

iv. Labium (lower lip) : 

• It is also called as second maxilla which covers the pre-oral cavity from ventral side and is firmly attached to the posterior part of head. 
• It has labial palps which is three jointed and sensory in function. 
• It is useful in pushing the chewed food in the preoral cavity and also prevents the loss of food falling from the mandibles during the chewing.

v. Hypopharynx (Tongue) : 

• In front of the labium and between first maxillae, there is a some what cylindrical single structure called hypopharynx or lingua. 
• A salivary duct opens at the base of this structure. 
• Lingua bears comb-like plates called super-lingua on either side. 
• It is useful in the process of feeding and mixing of saliva with food.

02. Thorax : 

• Thorax is three segmented. 

1. Anterior segment is prothorax
2. middle mesothorax and
3. posterior metathorax. 

• Thorax bears three pairs of walking legs ventrally (one at each segment) and two pairs of wings dorsally (attached to mesothoracic and metathoracic segment).

Legs : 

• Three pairs of walking legs are present on ventral side. 
• Each leg has five podomeres namely-

1. coxa
2. trochanter
3. femur
4. tibia and 
5. tarsus. 

• Tarsus is the last segment and is made up of five movable segments or tarsomeres. 
• Last tarsomere bears a pair of claws and cushion like arolium helpful in clinging.

Wings : 

• Two pairs of wings as forewings and hindwings are present on dorsal side.
• Forewings are first pair of dark, opaque, thick, leathery wings which are protective in function.
• Hindwings are thin, broad, membranous, delicate and transparent. 
• These are attached to tergum of metathorax. 
• These are helpful in flight and hence are also called as true wings.

Spiracles : 

• These are a series of slit-like openings on either side the body. 
• In all, there are ten pairs - 

1. two on thorax and 
2. eight on abdomen. 

• The spiracles let the air into and out of the tubes called trachea.

03. Abdomen : 

• The abdomen is elongated and made up of ten segments. 
• Each segment has a dorsal tergum and ventral sternum.
• Laterally, tergum is jointed to sternum by soft cuticle called pleura. 
• The posterior segments are telescoped in. 
• Due to this, eighth and ninth terga get overlapped by the seventh. 
• The tenth tergum projects backward. It is deeply notched. 
• The tenth tergum also bears a pair of small, many jointed anal cerci.
• In the male, the abdomen is narrow and tapering than that in female. 
• In male, the ninth sternum also bears a pair of short, unjointed anal style.