Through a Pocket Lens

CHAPTER VI.. Aquatic Insect Larvae 157

AQUATIC INSECT LARVAE 157

INDEX 189

LIST OF ILLUSTRATIONS

FIG. PAGE

Ptychoptera paludosa. Limnobia replicata Frontispiece

1. Hand Magnifier and Stand 14

2. Zeiss’s Dissecting Microscope 16

3. Leitz’s Dissecting Microscope 17

4. Two Leitz Lenses in holder (open) 18

5. Two Leitz Lenses in holder (closed) 18

6. Home-made Dissecting Microscope 19

7. Beakers 21

8. Glass Capsule 21

9. Glass Block, with cover 22

10. Glass Box, with cover 22

11. Forceps 23

12. Three forms of Dipping-tube. Method of using it 24

13. Mounted Needles 25

14. Cape Peripatus (natural size) 30

15. Margined Water Beetle (male) 32

16. Shells of Molluscs broken up by Dytiscus 33

17. Outline of Dytiscus 38

18. Male Dytiscus in flight 39

19. To show fold of (right) wing of Dytiscus 40

20. To show fold of (right) wing of Dytiscus 40

21 and 21 A. Head of Dytiscus 42

22. Disposition of mouth parts 43

23. Leg of Cockroach 44

24. Tarsus of Dytiscus (magnified) 45

25. Female Dytiscus swimming 46

26. Upper surface of abdomen of typical Beetle 47

27. Spiracle of Dytiscus (magnified) 48

28. Tracheal tubes of Dytiscus (magnified) 48

29. Great Water Beetle 51

30. Female Hydrophilus constructing a cocoon. (After Lyonnet) 55

31. Cocktail Beetle 58

32. Cockroaches 66

33. Mouth parts of a Cockroach 69

34. Cockroach, showing Spiracles 71

35. Alimentary Canal of Cockroach 73

36. American Cockroach (male) 75

37. Larva and Pupa of Earwig 77

38. Earwig (male) 78

39. Great Green Grasshopper (female) 81

40. Tibial ear of Great Green Grasshopper 85

41. Land Bug (magnified) 86

42. Water Scorpion 87

43. Organs of Water Scorpion, Egg, and Parasitic Mite. (After Swammerdam) 90

44. Raptorial leg of Water Scorpion 92

45. Water Boatman 93

46. Water Boatman swimming 94

47. Corixa, with wings expanded 95

48. Scheme of under surface of Wolf Spider (female). Pedipalp of male (enlarged) 98

49. Garden Spider and Web 99

50. Threads of Spider’s Web 100

51. Anchorage of Web 101

52. Foot of Garden Spider 104

53. Spinnerets of Garden Spider 104

54. Jumping Spider 106

55. Falces of Male Jumping Spider 106

56. Foot of Jumping Spider. Scopula much enlarged 108

57. Diving Spiders 109

58. Cell of Diving Spider 112

59. Red Water Mite 114

60. Larva of Water Mite 117

61. Nymph of Water Mite 117

62. Beetle Mite 119

63. Lithobius forficatus. Mouth parts seen from below. (After Graber) 124

64. The Common Millepede 126

65. Segments of Millepede (magnified) 127

66. Prawn 132

67. First walking leg of Shrimp (enlarged) 134

68. Mysis, or the Opossum Shrimp 135

69. Maxillipedes and Maxilla of Shore Crab. (After Savigny) 138

70. Stomach of Crab laid open 139

71. Gammarus. (After Sars) 142

72. Maxillipedes of Gammarus marinus (magnified) 146

73. Nest-building Amphipod (from life) 148

74. Water Woodlouse 153

75. Mouth-lock. (After Burgess) 161

76. Dytiscus Larvae 162

77. Pupa of Dytiscus 164

78. Larva of Limnobia replicata 167

79. Forked spine of Limnobia (enlarged) 168

80. Pupa case of Limnobia 169

81. Fore wing of Bee, showing marginal fold (×7) 170

82. Larva of Paraponyx stratiotata (enlarged) 173

83. Diagram of segment of Paraponyx, showing arrangement of tracheal gills 175

84. Gill of Paraponyx larva. (After De Geer) 176

85. Larva of Sialis (enlarged) 179

86. Diagram of Sialis larva, showing arrangement of gills 181

87. Pupa of Sialis 181

88. Larvae of Ptychoptera paludosa (from life) 184

89. Ptychoptera Larva (enlarged). Tail. (After Lyonnet) 186

90. Pupa of Ptychoptera. (After Lyonnet) 187

THROUGH A POCKET LENS

CHAPTER I. The Pocket Lens, the Dissecting Microscope,

THE POCKET LENS, THE DISSECTING MICROSCOPE, AND SOME SIMPLE APPLIANCES

The object of this little book is to show how much may be seen with an ordinary pocket lens, and with a simple microscope; and, as far as possible, to dispel the idea, far too common, especially among beginners, that no real work can be done unless one has a compound microscope, with a large battery of lenses and an array of ‘accessories.’

It would be easy to multiply quotations, from high authorities, in support of the proposition implied in the foregoing paragraph. Two only must suffice.

In a recent review of a very good book dealing with Butterflies and Moths (Natural Science, vol. vi. p. 293), the following passage occurs: ‘The only suggestion we should like to make is that a compound microscope is unnecessary for any of the details that the author mentions. A first-rate platyscopic hand lens is much more convenient and the young naturalist should train himself thoroughly in the use of it. There is no more common error than the undue use of the higher powers of a microscope. Except for the intimate details of histology, a low power or a hand lens is much more easy to use, and its employment gives a much better idea of the structure.’

The next quotation is of greater interest, as it gives some insight into the way in which Darwin carried on his investigations. In the Life and Letters of Charles Darwin (vol. i. pp. 145, 146) we are told: ‘His natural tendency was to use simple methods and few instruments. The use of the compound microscope has much increased since his youth, and this at the expense of the simple one. It strikes us nowadays as extraordinary that he should have had no compound microscope when he went his Beagle voyage; but in this he followed the advice of Robert Brown, who was an authority in such matters. He always had a great liking for the simple microscope, and maintained that nowadays it was too much neglected, and that one ought always to see as much as possible with the simple before taking to the compound microscope. In one of his letters he speaks on this point, and remarks that he always suspects the work of a man who never uses the simple microscope.’

It may be well here to verify the quotations, and also to consult Darwin’s Naturalist’s Voyage, to ascertain what kind of objects he examined with the simple appliances at his command. In the first chapter there is an interesting account of a curious limy deposit on the rocks of the island of St. Paul’s, and of the discoloration by confervae of the water, which, ‘under a weak lens, seemed as if covered by chopped bits of hay, with their ends jagged.’ Then we have an account of the confervae in the Indian Ocean, and of infusoria so numerous as to tinge the water off the coast of Chile. In the second chapter we have observations and experiments on planarian worms. ‘Having cut one of them transversely into two nearly equal parts, in the course of a fortnight both had the shape of perfect animals.’ In the next chapter he records some observations on the structure of vitrified tubes formed by lightning striking loose sand. In the fifth chapter is an elaborate description of a kind of sea-pen; and in the ninth chapter there are some remarks on the vast number of eggs in the egg-ribbon of a sea-slug, and on the ‘bird’s-head’ organs in certain Polyzoa. These remarks were, of course, founded on actual inspection with the simple microscope.

To this instrument, also, we owe the discovery of the tadpole-like larvae of Ascidians, or Tunicates, as they are now generally called. ‘At the Falkland Islands I had the satisfaction of seeing, in April, 1833, and therefore some years before any other naturalist, the locomotive larvae of a compound ascidian.... The tail was about five times as long as the oblong head, and terminated in a very fine filament. It was, as sketched by me under a simple microscope, plainly divided by transverse opaque partitions, which I presume represent the great cells figured by Kovalevsky. At an early stage of development the tail was closely coiled round the head of the larva.’

* * * * *

We come now to our pocket lens, which may be purchased for a few shillings of any optician. One can buy a serviceable single lens, in an ebonite handle, for a shilling; and this cheap instrument is sufficiently powerful not only to give the worker a good general idea of the form and structure of objects, but to enable him to do real work. With it the habits of many of the inmates of his aquaria may conveniently be watched; he may see their development from stage to stage of their life-history; and with it, when they are broken up, he may make out a good deal of their external and internal anatomy.

A very good form is shown at Fig. 1, which represents a hand magnifier, fitted with three lenses of different focus, generally 2 in., 1½ in., and 1 in. Examination of the catalogues of the principal London opticians shows that such a set of lenses may be bought for about 3s. In shape and construction there is sometimes a little variation; but the form figured is that most generally adopted, and is, on the whole, fairly convenient. It would, however, be an advantage if the hole by which the magnifier is mounted on the stand were drilled in the solid part of the handle. This would not only do away with the objection that the hole in the case permits dust to penetrate to the glasses, when carried in the pocket, but would give a longer reach, and thus obviate the necessity for moving the stand if the observer were examining a large object. The price of the stand figured is 2s. 6d.; and one with a short adjusting arm ought not to cost much more.

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