Locusts, also known as leeches.
Locusts are commonly known as annelids, belonging to the phylum Annelida. Unlike other annelids such as earthworms or sandworms, most locusts live temporarily as external parasites. Adapted to this way of life, locusts have no bristles on their bodies, and they have suckers at the front and rear ends. They have well-developed muscles and their body cavities are filled with muscles and connective tissues, making them smaller.
Locusts are a highly specialized group of annelids. There are over 300 known species worldwide. Most of them live in freshwater; a few are marine or brackish water species; some are terrestrial or amphibious; they are mostly found in warm and humid areas. The size ranges from 4-200 millimeters. Locusts have a ring-like structure and are hermaphroditic. Therefore, they are generally believed to have evolved from a group of oligochaete ancestors that adapted to external parasitism.
Many species of locusts feed on the blood of vertebrates or invertebrates. Some are more or less permanently attached to one individual, resembling external parasites; some only temporarily invade a host, suck blood, and then drop off; but there are also ordinary predatory or scavenging types.
Locusts are generally flat on the back and belly, with a narrower front end, and their entire body is leaf-like or worm-like. The shape of the body can change depending on the degree of extension or the amount of food intake. The body is segmented, and the anterior and posterior segments of the body have evolved into suckers. The anterior sucker is smaller and often surrounds the mouth; the posterior sucker is cup-shaped and mostly faces the ventral side. Except for polychaetes, the segmentation of the body is relatively fixed, indicating their close relationship and high specificity.
Leeches are the only group of annelids with a small and fixed number of body segments, making them more agile than most other annelids. The number of body segments is determined by counting the number of ganglia and lateral nerve branches. The body of all locusts is composed of 33 body segments, including the preoral sucker. Each segment is represented by Roman numerals I-XXⅪV (sometimes Arabic numerals are used for convenience). Each body part includes the same number of segments: 6 segments in the head (I-Ⅵ); 3 segments in the anterior part of the ring (Ⅶ-Ⅸ); 4 segments in the ring (X-ⅪH); 11 segments in the midsection (XlV-XXlV); 3 segments in the anal region (XXV-XXⅦ) and 7 segments in the posterior sucker (XXⅧ-XXⅪV). Since the 7 segments that make up the posterior sucker are difficult to distinguish, they are omitted when describing the number of segments of the locust, and only the body is divided into 27 segments. The surface of each segment is further divided into several body rings by transverse grooves. Three rings per segment is the basic type. Some segments of locusts do not have rings or have only 2 rings. However, most segments of locusts are formed by further division of these 3 body rings (represented by a1, a2, a3) into more rings-6 rings (b1-b6), which can be further divided into 12 rings (c1-c12) or 24 rings (d1-d24). However, most locusts only have groups a and b, and locusts with group d are very rare.
Generally speaking, the number of rings in the front and rear segments of the body is small, and the number of rings that make up the segments gradually increases from both ends to the middle. Many segments in the middle of the body have equal numbers of rings, such as 3 or 5 rings per segment (some may have more rings depending on the species), these segments are called complete segments. In contrast, the remaining segments at the front and rear ends are called incomplete segments. If a segment has more than 3 rings, the degree of division of these 3 original rings is usually different: some rings are further divided, while others are not or are less divided. Taking medicinal leech as an example, its complete segment consists of 5 rings, which is due to the fact that a1 and a3 each divide once, while a2 remains unchanged, resulting in the formation of b1, b2, a2, b5, and b6. The middle ring (a2) of each segment is the sensory ring, which contains the segmental sensory organ and the main branch of the lateral nerve.
Locusts have abundant sensory organs, with various types of single eyes, sensory papillae, and nipple-like protuberances distributed throughout their bodies. The eyes are composed of many photosensitive cells located behind the pigment cup.
The head is distinct. The eyes are on the dorsal side of the anterior end of the head, and their number, position, and shape are one of the distinguishing features of different species. The number and arrangement of eyes in some leeches may vary. The anterior sucker on the ventral side of the head can be prominent or just slightly thickened lips around the mouth.
The surface of the ring has openings of ring glands. During the breeding period, the ring becomes thicker and has different colors, making it easy to distinguish from the rest of the body. In the central part of the ventral surface of the ring, there is one male and one female genital pore. The male pore is in front, and the female pore is behind, with several segments in between. The number of segments between the two pores is usually 2 in the glossiphoniid leeches, 5 in the hirudiniformes, and 2-6 (or even 11) in the erpobdelliformes. There are also some species, such as the broad-bodied leech, where the male and female pores are combined on one circular protrusion. In some species, such as the pale-colored leech, the male and female pores are combined into one.
The central part is the largest part of the body, with a curved back and a relatively flat ventral side.
The posterior end of the body is composed of 7 segments that make up the posterior sucker, which are fused together and do not show any signs of segmentation on the surface. The last segment has an anus, called the caudal segment. The anus of a few locusts is located in the posterior sucker, but in most locusts, the old anus has been closed, and a new dorsal anus is formed just in front of the posterior sucker to replace it. The posterior sucker is a powerful suction cup that is indispensable for movement and feeding. The posterior sucker of locusts that suck human or animal blood is particularly well-developed.
Locusts generally respire through their body surface, and a few have respiratory organs on their sides, such as the respiratory sacs (contractile sacs) of fish leeches or the gills of branchiobdellids.
