Digestive, Circulatory, Respiratory, Nervous, Reproductive Systems of an insect (cockroach)

Introduction

Cockroaches are the most primitive living winged insects. They belong to the class Insecta of Phylum  Arthropoda. They are mostly found in damp places rich in organic components and in human spaces like stores and hotels. They are omnivores and feed on bread, fruit, cloth and even dead bodies. They are considered serious pests and vectors for several diseases. 

Morphology

The cockroach has a small head with a narrow, elongated, bilaterally symmetrical body divided into head, thorax, and abdomen. The head comprises sense organs: a pair of compound eyes, long flexible antennae and mouthparts with great mobility in all directions. Antennae bear tactile and olfactory receptors, which are sensitive to touch and smell. The mouthparts of cockroaches consist of labrum or upper lip, a pair of mandibles, a pair of maxillae, the labium or lower lip, and hypopharynx, which enables them to bite, chew food and lap up liquids. 

Jointed legs and two pairs of wings (from mesothorax and metathorax) are present in the thorax region while the abdomen consists of the anus (10th segment) and genital aperture (7th, 8th, 9th segment), which is surrounded by gonapophyses. Anatomy of a cockroach depicts a well-developed digestive, circulatory, respiratory, nervous and reproductive system. A brief description is given below.

Digestive System  

The cockroach has a well developed digestive system comprising the alimentary canal and associated digestive glands. Its alimentary canal is divided into three parts:

• The foregut comprises the mouth, pharynx, oesophagus containing salivary glands, and the longest part of the foregut i.e. crop, situated at the 3rd or 4th segment it stores partially digested food. Oesophagus opens into a sac-like structure called crop,which stores food.Crop  opens to gizzard which helps in grinding of food
• The midgut is a thin-walled coiled tube where food’s main digestion and absorption begins. It comprises six to eight finger-like structures called hepatic caeca(present at the junction of midgut and hindgut), which secrete digestive enzymes.Malpighian tubules are also present at the junction of midgut and hindgut. These malpighian tubules are rings of 100-150 yellow thread-like structures that help remove excretory products from the haemolymph
• The hindgut is broader than the midgut and comprises the ileum, colon and rectum. The rectum opens to the anus, which eliminates the faeces

Cockroaches are nocturnal organisms that search for their food with the help of antennae and taste it by maxillary and labial palps. They capture food with the help of forelegs, labrum and labium. In the mouth, the mixing of saliva and food takes place, which is then pushed to the pharynx with the help of the labium. Food starts moving through the alimentary canal by peristalsis. Food is partially digested in the crop which leads to the gizzard, where it is masticated by internal teeth. In midgut, the partially digested food is mixed with enzymatic secretions of hepatic caeca for complete digestion, and epithelial cells of midgut absorb the nutrients from digested food. Undigested food is passed first into the ileum and then into the colon. In the rectum, water is absorbed, and faeces are eliminated through the anus.

• Circulatory system 

It is of the open and lacunar type, which means blood flows through the internal open spaces and comes in direct contact with body organs. It consists of–

• Blood/Haemolymph consists of colourless plasma and haemocytes but is devoid of respiratory pigment, so it does not assist in respiration
• Haemocoel is the body cavity filled with blood, and the spaces are known as sinuses
• The heart consists of elongated muscular tubes lying along the mid-dorsal line of the thorax and abdomen. It consists of 13 funnel-shaped contractile chambers
• The anterior aorta is the first chamber of the heart and forms a single narrow tubular anterior aorta leading to sinuses (in the head region). The posterior end of each chamber has a pair of apertures called ostia. Heart receives blood through ostia guarded by valves that regulate blood flow. The blood from the heart is pumped into sinuses anteriorly by the contraction and relaxation of a pair of fan shaped triangular muscles called alary muscles

Circulation of haemolymph takes place by pulsation of the heart (50 times/min). Respiratory movements of the abdomen and movement of alary muscles increases the pumping force of the heart. The large body cavity is divided into two membranous horizontal partitions (dorsal and ventral diaphragm) and three wide and flattened sinuses. The dorsal pericardial sinus contains the heart, the middle perivisceral sinus contains the gut, and the ventral perineural sinus or sternal sinus contains the nerve cord. The sinuses intercommunicate by the pores present in the diaphragm. The haemolymph enters the heart through ostia from the pericardial sinus. When the heart is filled, it contracts in the systole phase. When it becomes relaxed, it is in its diastole phase. The next systole follows after a short interval called diastasis.

Respiratory System

The respiratory system of cockroaches consists of spiracles, a network of trachea and tracheoles. Spiracles or stigmata are 10 pairs of slit-like openings present on the lateral sides of the body. They are guarded by bristles or hair to keep out dirt. The opening of spiracles is called sphincters which lead to tracheal chambers. The trachea is branched and penetrates all body parts, which are further divided into different branches known as tracheoles. These tracheoles terminate into tissues that are filled with fluid at the distal end and play a significant role during the diffusion of gases. Harmonious contractions and relaxations of tergo-sternal muscles in the abdominal segments cause rhythmic compression and expansion of the abdomen leading to expiration and inspiration. O2 enters different cells and tissues and releases energy while CO2 leaves through trachea and spiracles.

Nervous System

The nervous system of cockroaches consists of CNS (central nervous system), PNS (peripheral nervous system) and SNS (sympathetic nervous system). It is spread throughout the body. A bit of the nervous system is present in the brain and the rest is present in the ventral part (ventral nerve cord) of the body. The brain is a super oesophagal ganglion that supplies nerves to antennae and compound eyes. The sensory organs of a cockroach include antennae, pupils, maxillary-palps, labial palps, and cerci. The most important sensory organs are the compound eyes which contain 2000 hexagonal ommatidia of the fused rhabdom type. The eye has a biconvex lens or cornea, which helps get several images of an object, and this form of vision is known as a mosaic vision with low resolution and high sensitivity. Such vision is common during the night (nocturnal vision).

Reproductive System

The cockroaches are dioecious (unisexual) animals. Male reproductive system consists of a pair of testes, vas deferens, ejaculatory duct, utricular gland and phallic gland. Female reproductive system consists of a pair of ovaries, oviduct, vagina, genital chamber and colleterial glands. Copulation occurs at night during the summer. The nymph, hatched from the ootheca, gradually grows into an adult through the process of moulting/paurometabolous.

Conclusion 

There are around 4600 species of cockroaches. The common cockroach is scientifically known by the name Periplaneta americana. It is an arthropod whose body is divisible into three distinct regions: head, thorax and abdomen. Pair of wings present in the thorax first pair of wings called tegmina comes from the mesothorax and the second pair comes out from the metathorax. Cockroaches have a well-developed digestive system that helps them obtain energy from digested food. The malpighian tubules present at the junction of the midgut and hindgut helps in removal of excretory products. The nervous system consists of fused ganglia, which help control and coordinate body function. Sexual reproduction occurs in cockroaches as they are dioecious creatures with separate sexes. Male species are longer in length than females.