Nematode Pests of Horticultural Crops and their Management (1+1)

General Morphology

SENSORY STRUCTURES: Though, a part of nervous system, sensory structures are made up of cuticle and hypodermis.The sensory structures receive stimulus and transfer it to the nerve centre.

Two types:
Exteroceptors - Peripheral in location; keep the nematodes aware of external environment.
Interoceptors - Internally located on the cuticular lining of digestive and reproductive system. Perceive stimuli of the internal environment of the animal and pass to nerve centre
1. Cephalic Sensory Structures:

• The lip region is typically hexaradiate with oral aperture (mouth) in the center; encircled by six lobe like structures referred as lips or labia. Two of these lips are sub dorsal, two are sub ventral and two are lateral in their location.
• There are 16 labial papillae present on the lips.
• Papillae are arranged in two circles; an inner circle of six papillae one on each lip towards the oral aperture and ten in outer circle.
• The arrangement of ten labial papillae in the outer circle; two are dorso-dorsal, two are latero-dorsal, two are ventro –ventral, two are latero-ventral and two are lateral in their location.

Function

• Outer labial papillae are purely mechanoreceptors also referred as tactoreceptors as they are sensitive to touch.
• Inner labial papillae are bimodal in function i.e. they act both as mechano as well as chemoreceptors.

Figure 5.3 En face view of head to show the location of labial (cephalic) papillae

2. Amphids: These are the paired structures located laterally in the cephalic region. They are of varied shapes in different groups of nematodes.
They are chemoreceptors in function i.e. they perceive chemical stimuli.

Figure 5.4 Structure of amphid

3. Phasmids: Paired sensory structures located laterally on each side of the tail and open to the exterior through a minute pore. Mainly chemoreceptors but may also perform some secretary function.

4. Deirids: A pair of small protuberances, one on each lateral side in the centre of lateral fields in the oesophageal region at about the level of excretory pore. They act as mechanoreceptors.
Other sensory structures are hemizonids and hemizonoins (near excretory pore), cephalids (one or two pairs in head region) and caudalids (one pair in tail region).

THE INNER BODY TUBE (ALIMENTARY CANAL)
The inner body tube forms the alimentary canal (digestive system) of nematodes.
Feeding Habits

• Soil-inhabiting nematodes can be classified on the basis of their feeding habits as microbivores, herbivores (plant parasitic), predatory or even animal parasitic including human parasites.
• The structural organization of their stoma/mouth parts differ as per their feeding habits.

Microbivores. Many kinds of free-living nematodes feed only on microbes e.g. bacteria, which are always extremely abundant in soil. In these nematodes, the "mouth", or stoma, is a hollow tube for ingestion of bacteria.

Figure 5.5 Head regions of microbivore, predatory and plant parasitic nematodes

Predators: These nematodes feed on other soil nematodes or other animals of comparable size. They possess an armed stoma havingdenticles.

Plant parasitic (Herbivores): This group includes many members of the order Tylenchida, as well as a few genera in the orders Aphelenchida and Dorylaimida. The mouthpart is a hypo-pharyngeal needle like stylet which is used to puncture cells during feeding. Fungivores that feed on fungi, also possess a stylet to puncture hyphal walls of fungi.