10 



THE AGEICULTUllAL NEWS. 



January 2, 1904. 



INSECT NOTES. 



Thrips on Cacao and Onions. 



In a ivceiit report the Acting Agricultunil 

 Iii.structor at Grenada dra^v^s attention to the increasing- 

 severity of the attacks of the cacao thrips. It is 

 stated that on many estates and cultivations the 

 ravages of this insect have been so serious as consider- 

 ably to decrease the yield. Vigorous action on the 

 part of planter.s in Grenada is most necessary. An 

 obstacle in the way of getting rid of this pest appears 

 to be the fact that it also attacks a variety of other 

 trees, wild and cultivated. Suggestions as to the 

 treatment of the cacao thrips will be found in the 

 Agricultural Xfws, Vol. II, p. 88; while fuller 

 information is given in the We)<t Indian Bulletin, Vol. 

 II, pp. 17G and 288. 



Thrips appeared last year and did some damage to 

 the onion crops in Barbados. The pest has again 

 appeared this season but has fortunatel}" done little or no 

 damage as yet. No successful remedy for thrips has 

 yet been found, but trials are being made of various 

 washes which, it is hoped, will prove eftective in 

 combating the pest. 



Some Peculiar Insect Structures. 



It i:s a general rule among all animals, except the very 

 lowest, that individuals are bi-laterally synunetrical. This 

 means that if a line were drawn down the back from front 

 to rear or from head to tail, the right side would be like tlie 

 left, and tlie organs and appendages of one side just like 

 those of the other. This is especially true of insects, and in 

 this groui' of animal life the exceptions to this rule are very 

 rare. 



Thrips, however, presents a very striking exception, or 

 a good example of asymmetry as it is called. This is 

 noticed in the mouth parts. 



The jaws of insects are at tlie sides of the mouth and 

 work laterally, meeting in front. There are two pairs, the 

 larger of which are mandibles and the smaller maxillae, the 

 upper lip (labrum) and the lower lip (labium) form the 

 upper and lower walls of the mouth. There are n>aiiy 

 modifications of these parts in difTerent orders of insects, but 

 these modifications rarely affect the symmetry. 



In the tiies these are so manifest as to form a regular 

 proboscis for sucking. In many bees and wasps the 

 mandibles are but slightly modified, while the other parts 

 form a proboscis which is used for sucking, but this is 

 accomplished by a long tongue which reaches to the toji of 

 the proboscis, and the process is more a lapping action than 

 real suction, as in the fiies. The butterflies and moths and 

 the bugs, such as plant lice, scale insects, lice, bed bugs, 

 etc., have real sucking mouth parts, but the larvae of 

 butterflies and moths all have strong mandibles for biting. 

 Beetles (hardV)acks), grasshoppers, cockroaches and pond 

 flies have typical biting moutli parts in larval and adult 

 sta£;es. 



In the case of the thrijis, however, we have a rather 

 large group of insects of world-wide distribution among 

 which as\-nuuetry of the mouth parts seems to be the rule. 

 On the left side of the mouth is a large mandible, while on 

 the right there is none or oidy the merest trace of a 

 rudimentary jaw, and the upper liii is modified accordingly. 

 The right half is well develojied, while the left is very much 

 atrophied. 



Although the right mandible is com|)aratively large, 

 the insect feeds by sucking and the mandible merely serves 

 for i)iercing the tissue and not for chewing. 



A very good description of the asymmetry of the thrips 

 with drawings is given by Dr. W. E. Hinds in his monograiih 

 on the Thvi^nnojitera of North America, iiublished by the 

 United States National Museum, Washington, D.C. 



Insects are normally unisexual, but cases occasionally 

 occur where both .sexes are united in one individual. This 

 is known as hermaphroditism and is the cause of another 

 form of asj'mmetry. It i.s especially noticeable in those 

 insects in which there is a marked difference in tlie form, 

 size or colour of the male and female. Professor John 1!. 

 Smith, in his Ecoiwinic Eiitomohj()ii, says that no true herma- 

 phrodites occur among insects, but since the writing of that 

 book several instances have been recorded and the sjiecimens 

 preserved in difl'erent collections where they may be seen. 



Instances are on record of liermapliroditism in moths, 

 the females of which are large, light-coloured, with slender 

 antennae, the males much smaller, darker-coloured with 

 feathery antennae. The hermaiihrodite found has one side 

 distinctly male and the other distinctly female : one side of 

 the body is dark with the small dark wing and the feathery 

 antennae of the male, while the c>ther is light with the large 

 light-coloured wings and slender anteimae of the female. 



The former of these two cases of asymmetry is very general 

 with the entire order Thysanoptera to which thrips belongs, 

 ami which is considered to be one of the most primitive of all 

 the orders of insects and one of the oldest orders geologically. 

 The latter, however, is only occasionally found and is not con- 

 fined to any order or group of insects, and must be regarded 

 as a reversion to tyjie, jiointing to a remote ancestry in which 

 both sexes were united in one individual, as in the case in 

 the present-day forms of snails, slugs, and earthworms, and 

 away from which in>ects, in common with many other forms 

 of animal life, have developed by processes of evolution. 



ONION SEED AS AFFECTED BY AGE. 



The following account of experiments in connexion 

 with the germination of onion seed is taken from 

 Amcricaii Garden iin/ of October 17 : — 



Since November 1, 189G, the Connecticut Station has- 

 examined samples of onion seed, both grown in that State 

 and in California. While the number of samples examined 

 of California-grown seed is not large enough to make a clo.se 

 comparison, it is (juite evident that a larger percentage of 

 the California seed germinates than of the Connecticut seed. 



It is also shown that onion .seed more than one year old, 

 as a rule, has much less sjirouting capacity than new seed. 

 Whether the plants produced from old seed are as vigorous 

 anil i)roductive as those from fresh seed is quite another 

 (piestion, on which laboratory germination tests can give no 

 light. The average sprouting capacity of four average 

 varieties, of which a considerable number of samples has 

 been tested, shows that the three Globe varieties and the 

 Wrthersfield lied are essentially alike in sprouting capacity,, 

 but the White Portugal api)ears to be distinctly inferior. 



