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Embryonic stages of the Australian plague locust
Stage 0
Stage I (10%)
Stage II (15%)
Stage III (25%)
Stage IV (30-45%)
Stage V (45-50%)
Stage VI (55%)
Stage VII (60%)
Stage VIII (70%)
Stage IX (85%)
Glossary
Acknowledgements
Further reading
Text and diagrams adapted from Wardhaugh (1973) by permission.
Figure 1. Embryonic stages of the Australian plague locust (Chortoicetes terminifera). The first illustration shows the entire egg while the other drawings show only the developing embryo.
Stage 0
Period of blastoderm development which precedes the appearance of the germinal disc.
Stage I (10%)
Formation and growth of the germinal disc at the micropylar end of the egg; yolk pale yellow in colour, with fine granular appearance.
a) Germinal disc roughly circular in shape.
b) Germinal disc pear-shaped but still undifferentiated.
Stage II (15%)
Separation into protocephalic (head) and protocormic (tail) regions but without signs of segmentation.
a) Embryo contained within the micropylar dome; ratio length/breadth < 2.0.
b) Protocormic (head) region extending along the ventral side of the egg, the embryo as a whole forming almost a right angle; ratio length/breadth > 2.0.
Stage III (25%)
The first signs of segmentation, and the inward turning of the thoracic appendages are used to define the beginning and end of this stage.
a) Gnathal and thoracic appendages present in the form of rounded protuberances.
b) Gnathal and thoracic appendages become elongated, segmentation of the anterior margins of the abdomen begins; antennae extend toward anterior margins of mandibular rudiments.
c) Gnathal and thoracic appendages project laterally from the embryo; abdominal region greatly extended and representing about half the total length of the embryo; four or five abdominal segments clearly discernible. Anatreptic movement begins.
Stage IV (30-45%)
This period covers anatrepsis.
a) Embryo begins to broaden; appendages become segmented and start to turn inwards and downwards. Development may be arrested at Stage IVa (approximately 30% of complete development) if the soil is dry, i.e. quiescent stage 1.
b) Legs turned in and becoming contiguous along the mid-line, and beginning to retract in the direction of the thorax.
c) Hind legs folded into an N-shape; appearance of red-brown pigment in dorsal part of the eye. Diapause may intervene at Stage IVc (approximately 40-45% of complete development) depending upon environmental conditions during egg maturation. Red brown pigment appears in the dorsal part of the eye in some diapausing eggs. In non-diapause eggs, kept moist, this stage is observed only rarely and the pigmentation of the eye is absent.
The second moisture-induced quiescent stage may intervene at this point if the soil is dry, i.e. quiescent stage 2. Eggs can survive either in Q1 or Q2 for an entire winter and for 2-3 months in summer. Only a few eggs seem to survive a full summer without rain, but such survival might be important in allowing locusts to survive drought.
Stage V (45-50%)
Blastokinesis - a rapid process, the initiation being marked by the rupture of the serosa. Embryo arches backwards and rotates around the micropylar end of the egg
Stage VI (55%)
Blastokinesis is complete; the embryo occupies about half the length of the egg, and almost the whole of its width; the dorsal section of the eye becomes strongly pigmented; the remnants of the yolk-plug fill the anterior half of the egg and extend down its ventral side to the posterior tip of the embryo's abdomen.
Stage VII (60%)
A period of rapid growth during which the yolk is completely engulfed and the embryo extends to fill the entire egg.
a) Embryo occupies from half to three-quarters of the egg; the segmentation of antennae and labial palps begins.
b) Embryo occupies from three-quarters to the whole of the egg; the dorsal closure, which begins at the tip of the abdomen, is now in progress.
Stage VIII (70%)
The embryo fills the entire egg, dorsal closure is complete, but the engulfed yolk is still clearly visible.
a) Hind femur extends to about the fourth abdominal segment and is now shorter than the tibia.
b) Hind femur extends to about the eight abdominal segment; dorsal spurs of the hind tibia are well developed but not yet pigmented.
Stage IX (85%)
The first-instar cuticle is laid down and undergoes a progressive increase in pigmentation prior to hatching.
a) Pale orange-brown pigment appears around the median ocellus, along the inner posterior margins of the antennae and on the externo-median area of the hind femur.
b) Pigment spreads over the frons, the antennae and the genae, and also develops on the prothoracic and mesothoracic legs; the genitalia and the femoro-tibial articulations also begin to darken. The tibial spurs are developed but not yet pigmented.
c) Manibular cusps and tibial spurs begin to darken.
d) Tibial spines and tarsal spurs darken; the embryo is ready to hatch.
Glossary
Anatrepsis/anatreptic movement: movement of the embryo away from the posterior pole of the egg.
Blastoderm: layer of cells that surround the yolk at the early stage of embyronic development.
Blastokinensis: active movement of embryo where it passes from the ventral to the dorsal side of the egg and at the same time revolves 180 degrees on its long axis.
Diapause: a state where growth is arrested by an indirect effect of the environment. In the Australian plague locust, eggs laid during the decreasing daylengths of autumn may enter diapause at late anatrepsis when temperatures are mild (less than 25 degrees Celsius) but not hot (greater than 32 degrees Celsius). Diapause results in adult locusts not being present during the winter when it is too cold in temperate areas and too dry in subtropical areas for growth. Diapause ends 9 (+ 2) weeks later (usually by mid-June) and development resumes as soon as moisture and temperature conditions are suitable.
Germinal disc: as a result of increased cell division, the blastoderm (see above) becomes thicker in the ventral region of the egg. This thickening is called the germ band, which develops into the future embryo. In the locust egg the germ band initially takes on the form of a small disc before becoming differentiated into a primary 'head' and ‘tail’ region.
Micropylar end: refers to the end of the egg where the micropyle, a specialised pore, is found.
Quiescence: a state where growth is arrested directly by either cold or dry conditions. Unlike diapause, growth resumes as soon as conditions are favourable. In the Australian plague locust, embryos absorb most of their moisture between 25 and 45% development and if the soil is dry at the time of laying, no moisture is available for development to proceed and growth is arrested near the beginning of the moisture absorption stage i.e. at about 30% development. This arrest at 30% development is called quiescent stage 1 or Q1. Embryos can also enter quiescence after diapause is complete. During diapause, development is arrested at late anatrepsis (40-45% development) and if it is dry at the end of diapause, development is arrested at quiescent stage 2 or Q2 and does not resume until rain falls. Eggs can survive in a state of quiescence over an entire winter and for 2-3 months during summer. Only a few seem able to survive a full summer without rain, but such survival may be important in allowing locusts to survive drought.
Serosa: layer of cuticle formed around the yolk by the blastoderm during the early stages of embryonic development.
Acknowledgements
The APLC thanks Dr Wardhaugh for permission to reproduce text and diagrams from his Ph.D. thesis.
Further reading
Chapman, R. F. (1976). A biology of locusts. The Institute of Biology's Studies in Biology no. 71. Edward Arnold (Publishers) Limited, Southampton. 67 pp.
Hunter, D. M. (1989). Temperature thresholds for development in diapausing eggs of the Australian plague locust, Chortoicetes terminifera (Orthoptera: Acrididae). Environ. Entomol. 18: 213-215.
Hunter, D. M. and Gregg, P. C. (1984). Variation in diapause potential and strength in eggs of the Australian plague locust Chortoicetes terminifera (Walker) (Orthoptera: Acrididae). J. Insect Physiol. 30: 867-870.
Wardhaugh, K. G. (1973). A study of some factors affecting eggs development in Chortoicetes terminifera, Walker (Orthoptera: Acrididae). Ph.D. thesis, Australian National University, Canberra.
Wardhaugh, K. G. (1978). Description of the embryonic stages of the Australian plague locust, Chortoicetes terminifera (Walk.). Acrida, 7: 1-9.
Wardhaugh, K. G. (1980). The effects of temperature and moisture on the inception of diapause in eggs of the Australian plague locust, Chortoicetes terminifera Walker (Orthoptera: Acrididae). Aust. J. Ecol. 5: 187-191.
Wardhaugh, K. G. (1980). The effects of temperature and photoperiod on the induction of diapause in eggs of the Australian plague locust, Chortoicetes terminifera Walker (Orthoptera: Acrididae). Bull. Ent. Res. 70: 635-647.